Here is some logistical information concerning the composite UIM found on GM cars with the SII 3800 and options if and when you decide to replace it.
OEM/GM/AC Delco
GM UIM part number: 89017272
GM UIM gasket: 89017554
AFTERMARKET
APN 229679 <--Click for more details
Dorman/Motormite 615-180 <--Click for more details