Any chance you can get the part number off the ECU and the part number, or revision code, off the cruise control switch panels? They'll be 7 or 8 digit part numbers followed by two-letter revision codes. The ECU is probably P5604 and then (in bigger lettering) 3 numbers and two letters, the switch panels will be AB, AC, or AD revision code afaik. Between 98 and 99 they switched the resistance of the switch panels and only changed the revision code, and using an ECU from one side of the split with switch panels from the other side results in no cruise function. Fortunately it's as simple to fix as swapping to the right year range/revision code switch panels.
Reminds me, when you push the buttons does the cruise indicator on the dash come on? If it comes on, the wiring from the ECU to the cruise switches is functional and the switches and ECU are from the proper year ranges, and you have a brake light switch / vacuum / vacuum solenoid / vacuum solenoid wiring issue. If it doesn't come on, you almost certainly have an electrical issue between the ECU and the cruise switches, OR the ECU doesn't want to go into cruise mode for some reason, I know when I had a blown head gasket in my 98 that was resulting in a set CEL and/or flashing CEL, it often would refuse to turn cruise on.