Hi Russ, is the ignition turned to on when trying to shut the doors? They will open without the key, just with the lights turned on, but you need the key in the on position to supply the power for them to close. Obviously if one is closing ok, it's probably not this that's causing you the problem, but that's not to say that the wiring is right & not causing something anomalous with it. If this does not solve the issue, check the fuses (the silver box with two threaded posts) & the relays themselves.
Also there is a diode under the dash which could be checked. The diode is situated under the dash by the ignition. Again if one door's working ok, then it's probably not that, but might be worth checking just in case. The trouble is, is that you don't know if it's all wired in correctly & working as it was intended to, so it's just as well to check.
Here is a link that might help
http://www.gvtc.com/~rlaa/.
Assuming that the wiring is correct, inside the car up on the bulkhead near the brake pedal you'll find a blue and brown wire that run together as a pair through a gromett in the firewall. There's a diode on the brown wire but it's right up under the dash. Get a test light or meter & see if you have 12V on the brown when your key is "ON" and nothing when your key is off. If you have this, your diode is probably OK.
Also check the earth of the wires to the door, as a bad earth could cause you a possible problem. Remember, it needs the key turned to on to shut them, not to open, so a bad earth can screw things up here. Also as stated in my first post check that it's not just down to a stiff mechanism holding it open. hth
cheers....Nige