If you can read and have ANY mechanical ability, you can likely "fix" this
First, try holding the key to the start position, and pulling the shift lever from park through neutral and back, and see if anything clicks, or it tries to crank
Then go to MyMopar and download a factory service manual, which has a wiring diagram in section 8. You can also download aftermarket, simplified diagrams from the same site. These are sometimes easier to follow, but now always complete
http://www.mymopar.com/index.php?pid=31
"The way it works" is this:
The neutral safety switch on the transmission goes to ground in park or neutral. I don't remember if the 68 switch is one or three terminal, but the CENTER terminal is the NSS. Early models are one terminal, NSS only, newer models are three terminal, the outer two being the reverse lamps switch
Look at the starter relay:
The two "push on" terminals are the magnetic coil. One of these receives the "start" voltage from the ignition switch, and the second terminal goes down the firewall and connects to the neutral safety switch on the transmission.
WHAT YOU NEED to work on "this stuff."
A digital multimeter, available at any parts / hardware store or even Sears
A bag of alligator jumper leads
A 12 V test lamp, from any parts store
Also a good idea is a spark tester, for ignition / spark problems
Disconnect either "push on terminal" LOOK at the wiring, and determine which one runs down the firewall. The remaining one should come from the ignition switch
Put your meter or lamp on the "other" wire and twist the key to start. You should show voltage, 12V. If not, try the other wire.
Leave the wire which is energized in start (assuming it is so) .......leave that connected to the relay.
Make CERTAIN the transmission is in park, take an alligator lead and ground the remaining "push on" terminal of the relay.
What you are doing, is simulating the ground on the start relay
Twist the key, see if the engine cranks. If so, the trouble is either the wire / connectors going to the transmission switch, or misadjusted transmission linkage, or a bad switch, or something wrong internally in the trasmission "rooster comb."
If it does NOT crank, and you followed directions, then the relay is bad.
Take your multimeter and check continuity of the two "push on" relay terminals. You should show continuity, the magnetic coil.
Also the contacts can be rusted up / stuck in the relay
