What determines the type of bulb a car uses?

[Grazi07]

I've decided to attempt to make my own projectors. I found these on ebay
http://cgi.ebay.com/ebaymotors/ws/eBayI ... gory=36476

or something like this

http://cgi.ebay.com/ebaymotors/ws/eBayI ... gory=33710

and am planning to retrofit them into the existing unit. My only hang-up is whether or not my bulbs (9003) will fit in them. Is the type of bulb a headlight uses determined by the power supply or by some dimensional aspect, or even by the adaptor the bulb fits into in the lamp housing. Any info would be appreciated (Kevin?!?)

Thanks
Andy

[pythonjosh]

Do you have a gen 1 or 1.5? If you're a gen 1'er like me, go with the bixenon BMW (with angel eyes). If you have a gen 1.5, you'll have a hard time figuring out how to hook it up. And besides all this, it would be good to install a HID kit at the same time. And projectors don't need a reflective housing, so I'd cover up that shiny crap with whatever color your car is. I think that would look cool.
Josh

[Grazi07]

thanks for the input, and thats more or less what my plans are. I just didn't wanna get stuck with something that wouldn't fit the current bulbs.