Power, ground and VSS signal I assume.
Obd1.5 is actually a misconception. On ur car you have an obd1 pcm and an obd2 style connecter.
It might help to take a look at the adding a hud how to threads. As mentioned, all the wiring going to a hud before 04 was analog except for the radio. I believe that was data.
There's a 97-03 hud in my 00 Regal.