I have HID in my Spider
Hey John:
I have done the conversion to my 78 Spider. Cost me around $500 shopping off ebay. You must acquire new headlight housings that accept an H4 bulb. I am not sure on a GTV but I believe its 5 1/4 housings. I also don't know the setup on that car in regards to dim/bright. Does it use all four lights, two for low, two for bright or just the outer two housings for both bright and dim?
For a four headlight system, you can replace your low beam housings with the HID system and maintain your other housings for brights.
For a two headlight system you have two choices. You can get two types of HID systems. One type is essentially low beam only and would eliminate your high beams. The second type has a magnetic reflector that opens and closes with your low/high beam switch to essentially give you low and high beam lights while the bulb never changes intensity.
All of these are easily installable at home. They actually do draw less power than stock bulbs (HID is 35W vs. Halogen 55/65W). Tremendous light given off by real HID so aim the lights low. Color is determined by the temp rating on them. The system plugs into your old bulb harness not to power the bulbs, but as the on/off for the system. Requires the mounting of the ballasts and relays, and a seperate power source and ground for the ballasts and bulbs that is relayed on/off via your old harness.
A few words of advice. When choosing a HID system, choose hella/osram/sylvania for your ballasts and bulbs. Although the kits are put together by other manufacturers, they are the industry standard for the best components.
Don't be fooled by the $20-$50 Xenon filled halogen bulbs. They are coated to produce the color and produce less light in some cases, and worst of all they can draw as much as 110W which will pop fuses and maybe fry the harness.
John M