[Sandy]

1998s were built on the commercial chassis (intented for hearses, flower cars and ambulances) and therefore were found to have a poor quality ride, as in bumpy, hard, and unforgiviing. The underpinning were so beefed up with so much heavy duty that it was much more than needed. To correct this, they built the 1989 on a stock chassis and they ride much better. I am not sure that the switch was exactly with Job 1 of the 1989 cars. To know for sure, they used different VIN prefixes. A "Heavy Duty" car will have a VIN that starts:- 1GEEH while a regular chassis will have a VIN that starts 1G6KD.
Nicest one I ever saw is that Balboa Blue one, above. It's a 1G6KD car and the rarest color made (1 of 2). Has Skirts & nice color-keyed vinyl top, looking "Factory" rather than a tacky looking Sim-Con.