The last full ambulance was a '79 Superior, as mentioned bought originally by Roger White and now owned by Dave Richards. M-M built their last Cadillac ambulance in '78, and S&S built their last Cadillac-chassis unit in '76. Superior and M-M continued to briefly build box ambulances into the early '80s, and S&S actually built a few ambulances after '76 using the GMC "Transmode" motorhome body.
There were a small number of combinations built after '79 - as Jerry mentioned, he has one. Superior built a few Cadillac-chassis lowtop ambulances for export (to the Middle East I believe) as well.
Steve Loftin can clarify, but I believe CRS Coach in Oklahoma built a mid-'80s Ford wagon hightop ambulance.
While the van and box-style ambulances were becoming more popular, a few of the professional car ambulances remained in service well into the '80s or later. The last professional car ambulance in service here in Maryland, a standby unit at a race track, was retired in 2002. (Of course, I bought it.)
Interestingly, I had several conversations with Bud Smock, founder/president of PL Custom ambulance, about the possibility of them building a new Cadillac-chassis ambulance around 2005 or 2006. Alas, when he died, he took his idea with him. (Bud had also designed the box ambulances on Cadillac chassis built by Franklin for some NJ squads in the late '50s.)