I'm having issues with supplying power to my carputer, and I'm wondering if anyone has any ideas as to what might be wrong.

Some background on the install:
The carputer is installed in a 98 Jetta.
I've got an 8ga power cable running directly from the battery, and I've got a good ground on the chasis of the car. The power supply is the PW70a connected through the ITPS.

What's wrong:
When I start the car normally (without the carputer installed), my head unit beeps and I get an annoying 'buckle your seat belt' warning. When I install the carputer, about 80% of the time, the seat belt warning signal beeps once and dies, and my head unit gives me the 3 'warning beeps' which is usually plays when the unit has been powered off. The carputer will usually start to boot (ie: it posts and the character LCD backlight lights up), but the harddrive appears to be short on power and the OS won't boot.

I'm thinking about taking my car to the Sears battery place down the road, they do a battery/alternator diagnostic for $10, but I'm wondering if there's anything I'm overlooking.

Oh, and one more question:
Should I attach my ITPS to the 12v switched, or the head unit's remote line?


Thanks guys,
Kyle