How hard is it to change a starter in a 340 Duster?

If your dealing with an automatic transmission:

Could be a wiring/ neutral switch problem. Here's how this works:

One wire from the ign switch feeds power to the starter relay coil, but it must receive a ground from the neutral safety switch in park/ neutral.

Try wiggling the shifter from park to neutral while holding the key in "start."

On your starter relay, there are two push-on terminals. ONE of them is hot from the ignition switch "in start." Unplug one wire, check both with a test light to see if one is in fact hot when the key is twisted to start.

Then hook up that wire, and use a clip lead to ground the other exposed relay terminal. The car should now crank.

If it does, the problem is in the neutral safety switch or the harness down to it.

Now you'll have to get under the car, find the neutral safety switch near the shift linkage on the driver side near rear of trans. I believe you have the single terminal switch. Clip lead the terminal to ground, hook up the connector up at the relay, and recheck if the engine will crank

Be careful making these checks, and unplug the coil high tension wire, and clip it to ground. The car will CRANK IN GEAR during these tests

If the engine cranks, above in the test, either the trans linkage is out of adjustment, or the switch is bad. I've read, on the www, that finding the older single terminal switch is tough. You can substitute the newer three terminal switch, just use the center terminal. You'll have to buy a pigtail for the correct connector as well.