Simply Do the following (from GTR):
High Mounted Stoplamp (86.41.32)
Removal
1 . Disconnect the battery ground cable.
For additional information, refer to Specifications
2 . CAUTION: Always protect paintwork and glass when removing exterior components.
Remove the high mounted stoplamp. Carefully cut through the sealant using a glazing knife.
Disconnect the electrical connector.
Disconnect the washer jet hose.
3 . NOTE:
Do not disassemble further if the component is removed for access only.
Remove the washer jet.
Installation
1 . Install the washer jet.
2 . Carefully remove the sealant from the body to leave a smooth surface.
3 . Install the high mounted stoplamp. Check for correct alignment.
4 . Remove the high mounted stoplamp.
5 . Apply etch primer to any bare metal.
6 . Apply primer over the etch primer.
7 . Apply activator to the high mounted stoplamp.
8 . Fit a pre-cut nozzle to the sealer cartridge, remove the lid, shake out the crystals and fit the cartridge to the applicator gun.
9 . Apply an 8.0 mm ( 0.30 in ) wide continuous bead of sealant, 15.0 mm ( 0.60 in ) in, from the outer edge of the high mounted stoplamp as shown.
NOTE FROM davez26: Picture did not come through, I'd basically replicate the sealant path from the old light.
10 . Install the high mounted stoplamp. Connect the washer jet hose.
Connect and secure the electrical connector.
11 . Connect the battery ground cable.
For additional information, refer to Specifications
Sounds like it is basically glued in with RTV. I hope that this helps.