On my 97 GMC which should be the same as yours. I ran my wires just to the
right of the engine. Just make sure that you are clear on the inside. Up
under the dash!!!!Above your brake and gas pedal. The hole I drilled I
couldn't put a gromet in it becasue the firewall is double thickness there
so I used a piece of rubber hose and ran my wires through it. You can seal
it in so don't move. I think a piece of 3/8 hose is fine. then if you put
wire sheeving over your wires just side it up to the firewall and its all
covered.

Under the carpet in my truck (not sure about yours) is a rubber plug with a
black wire going through it.  This is located on the passenger side about
where the passenger would put their feet when sitting in the seat.  The wire
is the radio antenna wire.  You could probably slide a few more wires in
through the hole or make new ones in the rubber plug for any additional
wires you need to run through the firewall.

Tony