Heat riser stuck

Chrysler dealers used to carry a special solvent to use periodically on the heat riser shaft to keep it free, according to the manual, but I have never seen it. I will probably try fireplace window cleaner (Ace Hardware) when I start driving my A's regularly.
Thanks Bill I bought a couple of cans of
Mopar MP-50 MP50

From E-bay, there was an older formula that has been recommended in the past but for our protection it evidently is no longer avail. This is what replaced it Ive been told.