to get a separate circuit that is hot with ign on, you can wire a relay in the same manner as would be done to wire an aftermarket fog light. the difference is the switch wire is connected to the fuse box in a position that is fused "hot with ign on" instead of to a toggle switch
-start at the battery by installing a 10 ga inline fuse or auto reset circuit breaker
-connect 10ga wire to the outlet of the fuse/breaker. as a hint, since you are at the battery anyway and will also need a 10ga ground, you could use a twinned wire that has a protective sheath and run a power and ground into the cab with one operation.
-run that 10ga wire through grommet/insulator in firewall/floor and bring to location of your choice near the end use (dash cluster) that is accessible for servicing
-install a relay (like one used for driving lights) in the same area for ease of servicing and connect the 10ga wire to the "power in" terminal (usually terminal 30)
-connect a smaller ga wire (14 or 16ga) to a source of fused "hot with key on" power and connect that wire to the relay coil/switch terminal (usually terminal 85)
-connect a ground wire to the relay on the other coil/switch terminal (usually terminal 86). the coil circuit of the relay is what turns the relay "on" and connects the terminals 30 and 87
-connect a 10ga wire to the outlet terminal on the relay (usually terminal 87) and this will become the power supply that is hot with key on. this wire can be attached to a bus bar or insulated stud to be used as a power supply that is hot with ign on.
this same scenario could also be connected to a heavy duty continuous use rated 100 amp relay, like ones used on trucks that have campers or trailers and use the relay to charge the camper battery when the truck is running.
see link for a circuit that is similar
http://www.accessconnect.com/fog_dri...hts_wiring.htm