jack the car up, get a BRIGHT flashlight/shoplight/dive lamp and start traceing where various things come thru the firewall, there are numerous penetrations. Try and find one. MOST cars I have installed sound systems on have one that is covered/filled with a rubber blank about where the clutch pedal is, relatively high up, right below where the floor board makes the turn back to the bottom of the dash plastic. Pop it out, punch a hole in it, feed your power wire thru, then use a rubber-safe adhesive to put rubber washers (like what you have in your hose spigot) on either side of it. Feed the amp end of the power wire thru the hole in the firewall till the wire is where you need it, then pass the wire thru the plug and seat it back in place.
