install related problem?

i installed a  deck and sub/amp setup in a 1994 chevy 1500 today.  about 45 min later i got a call at work from the customer saying the lights went out on the way home, wipers stopped, and the battery sad 9 volts.

i about sh*t.

i used a GM battey tap at the battery.... so thats basic enough.  i ran (fused) through the firewall (grommet) to the amp and grounded it from there.  so thats all normal.

i did the check in and out list, everythings was fine except for the check engine light on the dash and some minor broken trim peices.  i drove it out with the new deck on a local radio station and the subs thumpin.  i saw no voltage or battery indicator lights on and i even remember playing with the dimmer for the dash lights when i was getting out to make sure the interior lights were working (non functional lights with the door trigger.

all  i can think of is the alternator taking a dump.  i dont know if it was original (well over 100k miles).  maybe the new load from the amp stressed it while it was on the edge.  it sounds like the vehicle ran off the battery for a few miles then failed from not chargning.

any input?