Open the hood and look for the
engine compartment fuse block next to the driver's side firewall. Open the
cover in order remove and inspect the fuse closest to the radiator labeled ING
(Ignition) to see if the fuse connection is visibly
or blown. If it is OK open the fuse panel located at the left end of the
instrument panel inside the vehicle. To open, push the tab on the access door
to the left and pull the door forward. Remove and inspect the
two top right fused labeled IGN and PCM/IGN to see if the fuse
connection is visibly broken or blown. Replace any defective fuses and
try to start the vehicle. If it starts it could have been a bad connection or a
more serious problem requiring service if the fuse keeps blowing. If it does
not start it could be the ignition switch or starter.
You Kia Optima amp is in the trunk on the passanger side, You have to pull up the carpet.
Have you checked the fuses in the cab? Newer vehicles have 2 fuse/relay panels. One inside and one under the hood. There is a mixture of both large and mini fuses in this inside panel. It's just under the dash to left of steering column. The only saving grace is there is more room than in most vehicles. A Chilton/Haynes repair manual will give you, with pictures, the exact location. It will give you the diagram of what fuse does what as well. And troubleshooting section may lead you in other direction to fix your problem.
not sure which radio you have but there is an amplifier under the back seat and there are some fuses there you may want to check your owners manual for fuse information
Are you sure the head light switch is not the problem?
Changed battery but radio never came back on