I have read a few posts about adding a tv tuner to the D2. Is there only one tuner that will work or can another tuner be used or modified that will work with the head unit?

The GEX-p5700TV and the GEX-p6400TV can both be used without modification.

The GEX-p7000TV needs modification in order for it to work.

I have the 7000 and it took me a total of 10 mins to modify it. Actually, it took me more like 3 days and 10 mins to do it, because I'm a dumbass and messed up 100 times, but its a very easy modification.

Search the forum, gnatgosplat has a post about how to modify it.