[Sandy]

I know the answer.
The 98s' were all built on the Cadillac heavy duty coachbuilder underpinnings, and recieved the 6 bolt wheels among many other items. Many buyers complained that their cars did not ride "like a Caddy", and were stiff & unforgiving. So, for '99 they built them on the regular chassis, however they continued to offer the Cadillac installed "Coachbuilder's Package" as a NO CHARGE "option" ...

SOME (this indicates not a total listing) ... but SOME of the items in the Coachbuilder's Pkg. were the heavy duty 6-bolt wheels, heavy duty chassis, heavy duty braking, heavy duty radiator and heavy duty cooling system, sintered metallic brake linngs -rear, heavy duty power window motors, firmer shocks and heavy duty suspension, heavy duty turn signal switch, and more.... They can be spotted by the different wheels. Otherwise, no visual cues.