1. If not OEM for powered seats, there will be no prewire especially a 1999
2. Do the powered seats have the toggle switches for moving the mechanism?
3. When you state Dash Switch, you mean just a Toggle for 12 volt power on and off?
4. Dash Switch---- you mean the activation switch to move the seat?
5. If you have the complete Seat package including the activation switches mounted on the side of the seat, then the ONLY thing you have to run is a 12 volt Positive to the 2 wire pigtail for each seat and a Negative. Ground can be to the seat mount pedestal IF the pedestal mount bolts thru the floor are anchored to a steel support.
6. 12 volts can be either Ignition ON activated or continous 12 volts. Most OEM's are wires Positive HOT all the time. Most seats are on either a 15 or 20 amp fuse for each seat
7. IF your seat pedestal is a swivel, you will have to allow extra Positive and Negative slack wiring to allow the seat to swivel without pulling the wires loose.
8. Most seat mounts have a 3/8 thick washer(4 bolts and washers) between the pedestal and the bottom mount of the seat. Doug