Check here:
http://en.wikipedia.org/wiki/Mannix
Season 1 - customized Oldsmobile Toronado (Oldsmobile never produced a convertible Toronado)
Seasons 2–3 - Dodge Dart GTS 383 convertible
Season 4 - 1970 Plymouth Cuda 340 convertible
Season 5 - 1971 Plymouth Cuda 440 convertible
Season 6 - 1973 Plymouth Cuda 340 convertible (actually a 1970 updated with 1973 grill and tail lights)
Season 7 - 1974 Dodge Challenger 360 Coupe
Season 8 - Chevrolet Camaro LT