The vehicles used for squad 51 had the chassis donated by Chrysler, and the body built by Universal Studios. At the time, the LA County Fire Shops built the bodies and offered to prepare a vehicle for the series Emergency, but would need 6 months to build the body. This did not work for shooting schudles, so LA County sent blueprints to Universal and the props department built the squad in 2 weeks. In the first 2 seasons, Engine 51 was an actual in-service LA County Fire Engine (usually Engine 60, the one station to Universal, or Engine 127, the engine housed at Station 127 which served as the exterior shots for Station 51). In the third season, Ward LaFrance donated an engine to Universal since they were building LA County's Engines at the time and wanted one of their engines featured on the show.
Many times, manufacturers make deals to have their vehicles featured on differetn tv series. In Law & Order, Ford made a deal to feature Ford vehicles, so unless it was a scene which required many emergency vehicles, you would see Fords. Hawai Five-O has a deal with Chevy, which is why you see pretty much all Chevrolets, including the police vehicles, and the vilians driving something else.