Welcome, I’m a new guy here too, and am also in the process of building an 87 Ram 50. I’ve had mine for several years but actually only moved it into the shop and got serious with it a few months ago.

"......Other than junkyards where could I get interior replacement parts like wiper switch, dimmer knob and door arm rests....."

The wrecking yard, E Bay or possibly members here are probably going to be your best bet. For the dimmer knob you might find some kind of universal replacement from some place like ACE Hardware.....I've actually seem some there that are pretty close looking.

The really bad news is your wiper switch. It is actually a combination switch and contains the Headlight Switch, Headlight Dimmer Switch, Turn Signal Switch, Windshield Wiper Switch and windshield washer switch. They are available new from several sources……..prices seem to range in the $225-$275 range. I thought I’d pick up a new one just to have on the shelf….that is until I saw the price. I’m keeping my eye out for a good used one.