I am going through the same problem as you right now. I have my door skins off and in the process of fixing the 84 300D turbo.
What you need to do is take apart the door skins and look in there, when I took mine apart, I found this plastic white peice broken, so I needed to buy a new $1 peice of plastic right? WRONG, the dealer only sells the entire lock assembly for $120.00, so i ordered it and replaced it, but the door lock still didnt work like its supposed to, why? Because another metal spring was broken, Its hidden behind the door so you have to remove it to check it, so I got to order that from the dealer monday.
It could be a vaccum problem or a broken spring, or a broken peice of plastic, the only way to find out for sure is take the skin apart.
As far as the trunk, I replaced the vacumm peice but i still cant lock or unlock it with a key, so I need to get it fixed next.