You didn't say the year of the vehicle, but one thing common to older vehicles is that the insulation on wires in the wiring harness that ges through the holes in the body and door becomes chafed and cracked and shorts out. You may have a high-resistance short to ground. That's a short but not a "dead short".
The harness to the driver's door is the usual suspect, becaus thet gets flexed repeatedly every tim ethe car is used.
These types of shorts can often be found by removing the door panel, untying and "breaking into" the harness, seperating the individual wires, and checking continuity (in this case to ground) while flexing them. You'll want to have the battery disconnected when you do this, as the lock circuits are always "hot" even when the key is out. Sometimes the cracks and chafs can be visually seen on the wire insulation.
There is a Hypot (high-potential breakdown) test that can be run, but you probably won't need to go that far.
PS: I agree that the 20 amp fuse is a bad idea.