Japanese market STSs have a separate receiver for each wheel.
...that makes sense...
(I forget about the Japan/Korea exports, they were complete out of my mind

)
...yes, Japanese and Korean market STS are the only exports, that use the RPO-Codes AE5 (Lock Controle Remote Entry) and UH3 (TPMS system Var. 2)
...they got the same "domestic" 315 MHz frequency band RCDLR, but they use special/blue colored sensors (some sources say they are 433 MHz?) in the wheels (US/domestic use grey 315 MHz)...
...so that combination makes unique parts and separate receivers for TPMS necessary...
...the other exports (e.g. european, middle east) use code UJ9 for TPMS (05 - 09) and AU8 (05 - 06) UHB (07 - 09) for Lock Control...(all 433 MHz - rke and wheel sensors)
My 2006 european export STS got AU8/UJ9...and as I said...there are no separate receivers in the rear bumper facia...(it's only the RCDLR)...
...I was able to locate the -what I think they are- spots/pockets/provisions in the (05-07 design) rear facia for UH3 receivers...they are empty...
I also found some diagrams for the pre-08...like Darcy/ddalder remarks before...incl. a separate UH3-only-part, a so called "Tire Pressure Module" in the right side trunk area (a part my export also doesn't have...

)
...btw., the UH3 front receivers are located/attached below the headlamps...
...so, separate receivers (and the module) are for Japan/Korea exports only...05 -07 and 08 - 09...the other exports and domestic use only the RCDLR...
side note...the only part-# I found for extra receivers was listed as a UH3 part for a 2009 STS...#25858733