By 1977 Mercedes was back and incredibly the story was not inspired by the works team but by the British Mercedes importer preparing a 280E, even UK registered ULL 772R, and winning the London-Sydney Rally 1977 with it. Mercedes then entered four 280E on the Safari 1978 as full works cars, the result was however an embarrassment. So on their second works event, the non-WRC Grand Premio Sudamericana (turn Rally Argentina) 1978, they entered some huge 450SLC alongside the 280E and here the 450SLC proved faster and won. So for future years 280E & 280CE models only started occasionally as some kind of a B-team entry. 450SLC & 500SLC were basically the same car, reasoning and details you find in the according chapters. Curiously the "initiators" Mercedes UK importers went the opposite route, as they turned to the slightly smaller and lighter 280E only after an embarrassing test start of Tony Fowkes in a 450SLC on the 1976 Tour of Britain.
Anyway, following the 450SLC success in Argentina 1978, Mercedes turned to a proper and very serious WRC program. But all they proved was that they really had the wrong ideas about rallying. Just before the start of the 1981 season Mercedes pulled out and was never seen again. With the introduction of group A rallying a few 190E 2.3-16 appeared, which was actually a well handling, although underpowered RWD car, but these were in no case works supported and soon disappeared again.
Why Mercedes had the wrong attitude, why their WRC program was regarded as an embarrassment and why they pulled out very suddenly in disgust is already in detail described in the description sheet to the very characterful Mercedes 450SLC (group 4), so please look there for more detail and the wider, fairer picture. 
