A friend of mine has an 01 chevy tahoe that drains the battery every two days if it is not driven. He had a flip down tv installed and power inverter for a playstation, but the shop that installed them said they weren't the problem. What can we do to isolate the problem?

Any feedback would be appreciated. :)

If it started when they did the install I bet it's related. Do the old test light trick!. Clamp to the disconnected negative battery cable and touch the negative post then start pulling fuses. Have somebody hold the test light while you are pulling the fuses.

You can do it alone if you use a buzzer instead of a test light.

I'd bet a punch in the gut it's the inverter.

I told my friend to check the inverter and the audio/video stuff he added and after disconnecting them he said the negative battery cable still sparks pretty heavily. He thinks his onstar system isn't working properly. Could the onstar be a possible drain?

Anything is possible, and the only way to find out which, is to follow through with what 70c10 suggested.
On a side note, electrical questions WILL be allowed in this forum if they are remotely close to being either engine or chassis related. Josh has promised us a new forum, "electrical", and untill it arrives, I want to accomadate people with electrical questions.
After I posted the above, I found out the promised forum is now a reality. I moved this thread, with a re-direct. :welcome2:

does it by any chance have a Delco battery? if so swap it out for an Interstate.
I have found Delcos to not tolerate parasitic draws ANY higfher that 25MA (0.025Amps)