It sounds like a bad door lock actuator. They are not that hard to replace, Unfortunately over the course of 13 years and 2 foxes, I bet I have changed about 6. The hardest part is taking off the panel, lol. There should be lots of posts on how to do it. There is a tutorial at www.50resto.com with pictures. If you have any questions on how, just let me know. 
__________________ 1993 Mustang LX, Reef Blue
2.3 4 cyl 105 CID
Mods:215/60R15 Tires, 3G Alternator Upgrade, KYB GR2 Struts\Shocks, Redline Tuning Quicklift Plus, Removed Air Intake, K&N Air Filter,Tinted Windows, JVC KDAR3000 Arsenal Series HU, 2-10" 4000 MTX RT Subs, MTX RT202 Amp, JL 3.5, 6x8, and Pioneer 6.5 = 6 speakers, StreetGlow UnderBody Neons, Corbeau Sport Seats w/lumbar support |