The door locks will unlock but will not lock

Additional Info.
Perhaps it is different in my 99 Club Wagon than in a 98, but the 99 has no lock/unlock relays. If the Remote/Keyless Entry System uses relays for the locks, they must be built in solid state relays. The Lock Actuators (with a built in CB of unlabeled value, like the Headlight Switch) are wired directly to the switches.
The Ford system switch is normally in what I call a pass through position/mode.

There are 2 wires that are connected/pass-through ALL switches (and both actuator wires are connected to ground). When a switch is pushed, the wires become either 12V or Ground. All of the switches have to be present for the system to work. If there is a bad corroded/oxidized connection in one switch that may kill PART of the systems function (Lock+no-Unlock or no-Lock+Unlock). For me the trouble started when a fork lift hit the rear PL door switch and broke the plastic retainer hardware. Over time and shutting the rear door it would work its way ever-so-slightly loose and then it would typically lock, but not unlock).

Temporarily fixed by tightening the terminal contacts and taping the switch to the harness. Eventually spliced in a different harness. While looking for a replacement rear switch/harness, I pulled one that only had 3 wires instead of 5. It may have been from a conversion van and would have to be wired differently than shown in my manual.

I believe it would be at the end of the circuit instead of wired between the master and RH switch and would only be good for lock/unlock of the rear door only. In addition, the rear door could not be controlled from either of the other 2 front switches unless some additional wiring was added between the front switches and the back lock motor, but bypassing the rear switch. This may have been the way future models were wired, can't say.

Mechanic wannabe
RocketmAn

Wow, you must have been doing a related search or you have a really long reading list to hit this 2.5 years later! My reply sounds confusing to me now, but it's seemed a weird setup to me at the time and was hard to follow even with a wiring diagram in front of you. I was trying to add an external, disguised unlike switch that would substitute for a "hide-a-key" and open the van if I locked myself out. Can't remember if it would even have worked with a DPDT switch so I was looking to mount another of the original ford rear door switches right above a flexible piece of rubber covering a semi hidden hole (do it yourself weatherproof switch). Sadly that van bit the bullet when it hit 320K and I got smart and started driving company vans (mostly Sprinters).

Did some research recently into going the full blown mechanic route, but the first school a looked at had too many warning signs (UTI) and I also needed something to do in a relatively short time span. Haven't gotten around to asking a MAC or Snap-On guy what a typical starting setup for a mechanic costs. In between vocations now, probably going to end up in the oilfields for a while.