Our shower recently began to act weird.  One day we had to turn the single control knob much hotter to get the same temp that we did before.  Now, about a month after that, we have to turn the knob even further, and we have lost a lot of our hot water pressure.  The rest of the house seems to have normal pressure.

From research here and other places, I think that I need to replace the cartridge.  Here is where I run into a problem.  Our faucet is a Stanadyne from about 1991 I believe.  There is no back access to the controls from the adjacent closet, and the hole behind the metal faceplate is only about 4-5" in diameter.  There was a plastic plate that was on the back side of the hole, that enabled the front faceplate to secure to it, but that blocks the entire hole.

How do I go about replacing this cartridge?  I would really not like to break open the closet wall, or have to enlarge the hole in the shower wall.  Is there a way to remove and replace the cartridge without making it a major reconstruction project?

Thanks in advance for your advice!

Stanadyne bought Moen. And if you have one of their valves remove the handle and pull the chrome stop tube straight out. Turn off shut off valves underneath the shower valve.
Remove retainer clip (copper horseshoe shaped clip) and pull out cartridge with channel locks, pliers, or cartridge puller. Lube new cartridge with silicone jel and
Insert the cartridge with hole on the stem facing the 6 o'clock position, or down in a tub/shower.
Reinsert retainer clip and screw in handle assembly, followed by the cap or insert button. Turn water back on. When you have the cartridge out check the inlet ports for any trash and before you install the new cartridge turn on the water and chec k the flow from the open valve body. You're flushing the water supply lines. If there is a diminished flow then the blockage is most likely in the seat of the shut off valve. If there is no access hole for the shut off stops then your valves have integral screw driver stops located on each side of the valve body. Hot stop on the left, cold is on the right. They can be reached by removing the two screws and removing the chrome cover plate. Good luck, Tom