My 98 Mystique LS does Not unlock all doors from the door key, and it has factory keyless.

To find out if you have keyless, check the rear of the dash by the kick panel, you may see a white 2 pin plug with a brown and a white wire. It may be taped with paper black tape. If you have that, you are in luck, go on e-bay and find some remotes like this

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33723&item=7929661432&rd=1

and program it and enjoy.