IF
the 450's servo is the same type as my early 123 280CE (ACCII, a.k.a. Chrysler's Revenge, vertical buttons on control),
[If not, ignore my prattling below and proceed directly to next post...]
have hood open and be near servo with engine running as someone else turns off engine - you should be able to hear it running for a few seconds as it returns to "park" - at which position, by the way, it should be "failed" to the posit mentioned by Col Klink - at which you should still get warm water, and blowers/doors controls should be bypassed/routed to give you the DEF as an emergency fallback with high blower, up through def vents, and hot water.
Manual on the mother-type system at
http://imperialclub.com/Repair/Lit/Master/281/cover.htm
and procedural checkout at
http://skinnerbox.steaky.org/Service/W123/Main.html
try under M110, then "78 280CE" (the one I use)
under "climate control" - step through to checkout procedure without factory test unit. If followed in sequence, may help isolate area of concern.
And of course, there's the Unwired Tools solution at top of Forum page...
And some servo rebuilders out there...