Interesting thoughts. I have similar thoughts that an H3 would fry my housing. However as you pointed out, tailgaters are annoying and I would love to have something to fight back with. There are rear foglights available but they are big ugly things so far as I have seen, and I do not want to spend 30-40$ on a good set of smaller lamps that I could benefit from more by using them on the front of the car. I am looking for a cheap and easy and creative alternative.
For about 12 bucks I saw a strobe kit at pep boys. It had 2 strobe bulbs and a few different colored covers for them. It also had a speed control box which they would wire into which would then be sent up to power inside and comes with a small toggle switch. These are cool for recreation and I in special circumstances such as having a moron on your tail, you could flash them real fast to piss off the guy tailing you if not making him think you are a special law enforcement vehicle. These little things would fit very easily into the empty hole. I saw a strobe at target for 10 bucks that looked like a mini third brake light with a swivel base that could be stuck in the back window and would have a good chance of scaring a tailgater...however at the risk of a cop seeing you do it.
Lastly If I was to add an extra bulb (so-called fog lamp) I don't think I would want something as wimpy as a 22W brake bulb. What fun is that? Is there anything that is a bit brighter without cooking my tail lamps?

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Rob C
89 190E 2.6
Beige