Do you mean the entire car or just the engine?
FWIW according to Wikipedia the R18 diesel was introduced in 1980, so I do not know if a diesel R18 was even built in 1978:
1980 (July): Introduction of the 18 Diesel. It had a 4-cylinder 2068 cc 2.1 diesel engine (which produced 66 bhp (49 kW)),
Look here for info:
http://en.wikipedia.org/wiki/Renault_18
http://www.renault18.com/
http://ptikem.free.fr/indexgb.htm