I changed the battery on my 2002 trailblazer, because it had ran down and was old. I got a jump and it started. I took the old battery off and put on a new one. Everything works in the car, except the radio and it will not turn over. It will not even make a sound when I try to start it.

SOURCE: Electrical Problem for a 2004 Chevy Trailblazer

hate to say it but jumping it the fuse relays may have damaged the vehicles fuse panel. even though no fuses appear to be blown too much current through a regulated power source shorts out wires, connections, terminals and lord knows what else. only real solution i can give you is take that car to a mechanic. and give your son the bill.

SOURCE: 2002 Chevy Trailblazer won't start

check the crank shaft sensor

SOURCE: Chevy Trailblazer 2003 ran the battery down now wont idle

try changing the air filter ,i had a problem ,at ilde in drive my trailblazer would stall, changed the air filter now it's not stalling and got my power back ,there is a secondary air pump that i have to change ,
not sure were it is ? still not running perfect but it's running better .and my service light is on too.
i got it put on scope ,it's the pump .
not sure if this helps ,but i'm start to run in to problems too,i have a 2004 trailblazer.
i like to do fix things myself ,i hope i can.it's the cost if you know what i mean.

SOURCE: 2003 Trailblazer- no power after key stuck in ignition

Sounds like an antitheft problem I would start with the antitheft system. Usually when you only have one key all keys have to be programmed together even the original. When you have more than one it is easy to program additional keys.

SOURCE: 2002 Chevy Impala Radio Doesn't Work

Check the radio fuse? Box is drivers side end of dash(drivers door open)or pass side end of dash(passenger door open)
police models have a connection for the speakers (passenger floorboard under floor mat) to connect the speakers to the police radios.
If they had the speakers connected to their radios,they may have unhooked them and not connected them back to the radio.
The wires are brown,grey,blue and light blue.All into a blue connector.