Are you installing an XTS-2500 or an XTL-2500?
If it's an XTS and you just need to connect a charger or battery eliminator, use the cigarette lighter plug, or find another easy source of 12 volts inside. Sometimes you can find wiring bundled up for accessories you don't have on your vehicle. Pop in a fuse and reuse the circuit.
If it's an XTL-2500, then you can often follow existing wiring. Start from the engine bay side and look for a large wiring harness running through the firewall near the brake master cylinder. On the last bunch of Ford and Chevy's I've installed in, there's a nipple on there than can be cut off and it's a pass through for after market wiring. I've run as large as one #6 conductor through those, so the power cord for the XTL should fit just fine.