3rd Brakelight retrofit into pre '86 car?

[Aufbau Principle]

I have a late 3rd brake light assembly and parcel shelf out of a 325i that I want to retrofit into my '84, which doesn't have one. There are 4 pins coming out of the 3rd brake light assembly, 2 of which look like they have to do with the check panel, according to the wiring manual:



The 1984 brake wiring for comparison below. It looks like if I want to do it legit I put pins 3 and 4 from the brake light assembly between the 'brake lights fault indicator' and the 'rear lights check relay' and grab power from after the brake switch.



A concern I have is that the wire coming from the brake switch going into the check relay is larger on the 325i diagram. If I remember my electronics rules correctly, putting another load in parallel won't affect current in the other branches, so I'm not sure why that wire is bigger. It looks like the wires are the same size elsewhere.

Aside from the wiring size, how big of a pain in the ass is it to actually do this? Where is the relay and fault indicator located? The other option would be to just to wire the light in parallel with one of the brake light branches, which is what I guess is done with the aftermarket 3rd brake lights available. The only problem that option might have is pulling too much load on one of the wires, maybe an LED would pull less current than the incandescent in there?

Looking for some input from anyone who's done this or is good at electronics.
Cheers

[Mo Brighta]

Adding center brake

May I suggest the Hella 5238 center brake light? It is LED, draws around half an amp so totally safe to add it to the circuit. They stick to the glass at top or bottom, your choice, and look very stock - plain matte black housing. I am ordering one today, for an Euro E24 six. Europeans have no requirement for center brake light, they actually pay attention while driving.

I sell a few of these every year - they are neat because they can be adjusted to shine the light beam up or down over a range, based on your particular driving environment (cars, trucks, SUVs, whatever you see the most of in your neighborhood). You aim it directly into the eyes of the enemy to maximize your visibility. Gives them no chance of saying "I didn't see him..." should they bump you from behind.

Also nice and bright in the 'hot spot' - helps insure they don't follow too closely if you tap the brakes now and then, to get them used to your existence.

I also have a few tricks for the tail light itself, doubling the size and tripling the output of stock tail and brake lights- safely - using the original reflectors and high output bulbs, and a little wiring improvement. Check panel operation, lenses, reflectors are safe too. I have pictures...

Let me know if I can help.


Andy