Is this automatic or stick. If automatic, the neutral safety switch down on the left side of the transmission GROUNDS in park or neutral. This is connected to one of the small push-on "flag" terminals on the relay, which is the coil. The other flag leads (usually yellow) through the firewall to the ignition switch "start" terminal. Very very simple circuit. I would inspect the key switch and connector / terminals, and inspect the bulkhead connector, AFTER checking the NSS
Try wiggling the shifter which indicates a failing switch or out of adjustment or disconnect that wire at the relay and temporarily grounding it. CAUTION. Will start in ANY GEAR doing so
IF IT IS STICK and has an OEM starter relay, the one terminal is soldered to the case and the relay case MUST be grounded
Run over to MyMopar. You can download many years of service manuals, and 2 page aftermarket wiring diagrams over there