What happened here ?? fuseholder burned 88 300CE

[JOE49]

Fuse "D" melted plastic on fusebox. '88 300ce. 1st. Benz for me (112,505 miles). All fuses are new, copper (Flosser), OVP is new too (per forum advise), AC was converted to R134a at 85,000 miles by orig. owner at Beck Benz of N. Carolina. Was evacuated & re-charged at 112,982 miles due to AC blowing cold when first turned on, but after parking & re-starting no more cold. Now has 113,072 miles & fall is here, so tried heat & no hot. Looked under fuse cover yesterday & found melted plastic at fuse "D" . Removed screws of cover to inspect beneath & ran out of daylight. Am going over wiring diagrams found at Benzworld. What could have caused this ?? Where to start ???