What you need:
OtterBox Defender case - Price: free since you already purchased the case to protect your phone.
Samsung Vehicle Mount - Price: $4:90 with free shipping (buy the cheapest one - it does not need to be a model made for your phone)
Dremel/Saw/some other cutting tool - Price: Free (if you don't have one borrow one from a friend)
Super Glue/Epoxy/plastic adhesive - Price: $1
Samsung Car mounts are made of two pieces. The Suction Cup Stand, and the part that the phone snaps into. If the two parts are connected, take them apart.
Use a dremel tool to cut the away the phone bracket from the clip that attaches to the suction cup stand. The part you need is highlighted in this picture. Pictured is a mount for the Captivate, depending on which vehicle mount you purchased the bracket may look slightly different.
Step 3 (optional)
Remove the belt clip from the holster that came with your OtterBox Defender case.
See Step 1 in this instructables for a description on how to remove the clip
This picture is from the link above.
Glue the piece you retrieved in step 2 to the OtterBox Holster.
The OtterBox holster will now just snap on to the Suction Cup Stand
Put it in your car. Place Phone in Holster. And enjoy.