Prerequisite for this course is Flashers on the Cheap. Read that first before continuing here.

To do this the better way you should retain the original flasher unit for the safety feature where it would cease to flash if a bulb burns out or is disconnected. You would also need an additional heavy duty flasher unit (as previously described). You then also need a triple pole switch, or a single pole switch (possibly the existing "F" switch) and a triple pole relay. I also recommend an additional 15 amp supply fuse, or original type Lucas 17/35 fuse.

Output of the flasher unit can be permanently connected to terminal 1 on the relay box (or the "L" terminal on the original flasher unit, same wire). The triple pole switch should source power from any always hot circuit, such as the brown/blue wire on the ignition switch or the brown (or brown/green) wire terminal on the fuse box. This is where you can install the new fuse if desired. Outputs of the triple pole switch should go to the new flasher unit input and to terminals 4 and 8 on the relay box (or terminals L and R on the turn signal switch, same wires).

When you throw the switch the new flasher unit will get power, and both relays inside the relay box will be energized at the same time connecting all four corner lights to terminal 1, and you have operational 4-way flashers. (The brake light switch output is disconnected at the same time).

An alternative to the triple pole switch can be a single pole switch and a triple pole relay. If you do not have fog lamps on your MGA you can use the original "F" switch on the dash for the flasher switch. For this you need to connect One terminal of the new relay trigger coil to always hot power. The other terminal of the relay trigger coil should be connected to the single pole switchm which will be connected to ground on the other side. The three common terminals on the relay can be connected to always-on power. The three normally open terminals get wired to the turn signal switch L & R terminals and the heavy duty flasher unt input. terminal. Here the single pole switch and triple pole relay together do the same thing as the triple pole switch noted above.

There are a couple potential problems which might occur with this setup.

First, you may leave the ignition off when the 4-way flashers are activated. In this case when the HD flasher switches on there is a voltage feedback through the heater element of the original flasher unit. This supplies a somewhat reduced voltage to the ignition circuits, by virtue of the resistance in the original flasher heater element. This reduced feedback voltage may be enough to cause the fuel pump to cycle, and the ignition coil will draw some current (but would most likely not produce a spark). I did test this circuit once with four bulbs (no trailer connected and ignition off), and it worked (sort of), as the original flasher unit did not switch on. Feedback through the original flasher unit heater element did make the fuel pump click intermittently, so the ignition coil must also be tanking some power by that route. This may merit some additional testing, especially with more than 4 bulbs for load, but I do not personally recommend leaving it wired up this way. One solution to the feedback problem would be to install a LARGE diode on either side of the original flasher unit (not in the pilot light wire).

Second, you might switch on the ignition when the 4-way flashers are activated. When the HD flasher is energized and switches on, both input and output of the original flasher unit is the save system voltage, and nothing happens there. However the output power from the original flasher unit can (and probably will) interfere with operation of the new HD flasher unit. Under the right condition one or both of the flasher units could operate eratically, or the original turn signal relays could chatter. Otherwise in between flashes, when the heavy duty flasher unit switches off (remove system voltage from the output side of the original flasher unit), the original flasher unit will be momentarily connected to the load of multiple bulbs. In that case the original flasher unit might heat up and switch on before the HD flasher unit switches back on again. This would apply system voltage to the output side of the new flasher unit, which would momentarily defeat the function of the new flasher unit. When the original flasher switches off it will be a race to see which flasher unit is first to switche on again. Installing diodes in both flasher outputs would not solve these problems.

To avoid the problem of double flashers interfering with each other, or feedback into the ignition circuit, you need to disconnect the green wire from the original flasher unit and connect this terminal to a normally closed contact on your new triple pole relay (which would then also need to be double throw). You could otherwise use a triple pole double throw switch with no relay. Then when you switch on the 4-way flashers the input side of the original flasher unit would be disconnected from power or from the ignition circuit at the same time power is connected to the new HD flasher unit. Alternately you could use one pole of the double throw relay to disconnect output of the original flasher unit from TS relay terminal 1, and connected output of the new HD flasher unit to the same terminal.

In any case the dash indicator lamp will not work with the original flasher unit disabled. If you were creative you might add another pole to the double throw relay to make the indicator lamp work with the four way flashers, or install two diodes on terminals 2 and 6 of the original relay to feed the dash indicator lamp.