Have you been working on the system? Swapped parts or anything like that? Was it working previously and just stopped? Just curious if you might have put new combination switch on from a different truck - perhaps one from a non-intermittant truck? Or, if it has never worked for you, maybe the previous owner did that. There are a number of parts that get pulled from junkyards but they don't alway fit.
Anyhow, it sounds like you have two different problems. The intermittent (or lack there of) sounds like a problem in the switch itself - assuming you have the correct combination and wiper switch installed. So I would pull the whole combination switch off and have a look. I have posted some info on the wiper switch in this thread (skip down to post #15.)
http://www.mazdatrucking.com/forum/showthread.php/11111-Purchased-a-B2600i!
As for the parking problem. If you or a precious owner has ever had the wiper motor disconnected from the wiper transmission, then you have to make sure they are timed properly when they are re-installed/reconnected - just like valve or ignition timing. So what I would check there is to pull the the wiper motor and then cycle the motor several times with the wipers disconnected to see if the shaft stops in the same position every time…. just mark one of the splines on the shaft. If it parks or stops in the same position each time, then I would re-install it. Here is where you need to make sure it is properly timed. With the motor having parked itself for the last time, you need to also position the wiper blades at the bottom of the windshield (where you want them in the park position.) Then reconnected the transmission to the motor. Now both the transmission AND the wipers are in the park position. If you don't have the wipers at the bottom of the windshield when reconnecting, then the system is going to get a little wonky as for parking.
Also, check to see if you have any slop between the two wiper blades. If so, then you probably have a worn out bushing between the two - very common problem. It also doesn't hurt to pull everything out and lube things up.