The 84 and early 85 variety Regals were all built in Flint. The last 8 of the Vin EHXXXXXX or FHXXXXXX reflex this. The H designates Flint. Production was shifted to Pontiac mid year 1985 when they retooled Buick City to Build the new (1986) Lesabre.
The last 8 of the VIN on 85, 86 and 87 Regals built in Pontiac FPXXXXXX, GPXXXXXX or HPXXXXXX designate a Pontiac built car.
Late 85 Pontiac built TR's and GN's all had Powermaster brakes, Flint built 84 and 85's all had Hydraboost.
All the bodies for the Regal were built at the Fisher Body plant in Flint and then shipped to Pontiac for final assembly. Ever notice that the paint on the front clip is different from the body (original paint job of course)? That's why. Front end sheet metal was painted at Pontiac, Body was painted in Flint.
I often saw truck loads of bodies headed down I-75 to Pontiac. You could always tell the GN's and TR's because they had the mounting bracket for the Powermaster installed at The Fisher Body plant.