Thanks for the suggestions so far. I thought I had solved the problem, cleaned and lubed everything that could be causing me trouble, but then it happened again. The pedal itself moves up and down when you move it by hand, but does not return to the up position and you can't pull the cable back at the engine. Right now I've got a heater in the van and a light for warmth aimed up at the cover over the linkage. 

The plastic cover over the linkage underneath looks like there is a piece missing at the top so water could get in, but I didn't see any there. 

When I get home I'm going to try shooting some WD-40 down the cable sheath at the back end until it comes out the other side. 

But nothing really looked frozen. I wonder if it was something mechanical, i.e. metal sticking to metal and locked up rather than freezing, and it was just a coincidence that warming it helped.

Any other ideas?

Thanks,
Ed
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