I recently upgraded the rectangular taillights in my ERA FIA. Here are some of the things I found:
LED bulbs, because they are directional, will not work in the Lucas style lamp (the socket points up to the sky, instead of rearward). Halogen bulbs WILL work well in this style lamp.
Halogen bulbs are very hot, and very bright. I have not had any problems with halogen bulbs in my ERA over the past two years. The Lucas lamp is "open", with no back-can to it, which lets the heat out. If the back of your taillamp body is enclosed, then I would not recommend the Halogen bulb, due to the heat issue as Ross mentioned.
I changed to a new Superformance taillamp, which looks just like the Lucas rectangular light on the outside, but had a completely enclosed back-can to it, with two separate bulbs. The bulbs face rearward, and I THOUGHT I would be able to use the new LED bulbs (low heat, low energy consumption, reported to be very "bright").
Bench tests clearly demonstrated to me that the $10 LED bulbs available at local auto parts stores are NOT as bright as a STANDARD 1157 bulb! I did not try the $16 LED bulbs from Performance Unlimited, they claim to be brighter than other brands of LED's.
I rewired my car to separate the brake light function from the turn-signals. I now have a standard 1157 bulb in the upper position performing tailight and turn-signal functions, and a Halogen 1157 bulb in the lower socket performing ONLY brake light function. When I hit the brakes, it REALLY lights up back there.
I further increased the light output of the new lamps by enlarging the opening of the back-can retainer, AND enlarging the opening in my fender to gasket size ("ported" taillights?)
I think the hot Halogen bulb will be OK in the enclosed back-can, when used ONLY for a brake-light. I am afraid that it might get too hot (with an enclosed back-can) if used for the taillamp function.
Regards,
Jeff Burgy