Hold on, you haven't told us how old your GLS is. It makes all the difference.
If its a pre-facelift then you wont have the necessary socket for the wiring loom.
If its a mini-facelift then you may be lucky & it may have the socket in the passenger side kick panel for the loom to plug into.
I was lucky in that my '98 CD had the connector in place - but it was taped up and out of the way.
I think you might be very lucky indeed if it's there though as a mates '98 GLS dosn't have the wiring in place for heated seats and rear electric windows. So, fingers crossed.
If not then you need to hack off as much of the cable on the loom of the car that you are removing it from.
Some models ('98 I think) have one Carin loom that plugs into this main loom, whereas the newer ones have a two piece loom, with the dash portion being a real challenge to extract. If its a single loom model then there will also be a thin red wire which trails around into the back of the fuse box under the steering column. This was resolved with the newer 2 part ones.
Older ones have the GPS aerial visible and located near the tax disk, but newer ones have it hidden behind the centre air vents. The visible ones have a long thin coax to the drive wheras the 2 part loom ones have a coax joint.
If your car is a saloon then the mounting bracket for the drive unit may not be as shown in the picture, but one having long arms going to bold positions - recover the bolts/nuts as you will need them. If its an estate then it could be located in one of 2 other places, but we can go into that if needs be.
If you are tempted to install the drive unit elsewhere, then make sure that its horizontal with the CD slot pointing to the drivers side. The unit is fitted with gyros and needs to be this way round to avoid getting confused.
Finally, if you don,t have the connector then dont dispair, if you hack the connector off the donor car then you will have the basis to go forward - the necessary signals for the connector are easy enough to splice in.
Hope this dosn't confuse you too much

, I had to figure it all out from scratch when I did my first one many years ago before OOF existed.
HTH .......... Rob