Well it's supposed to flash a 2 didgit code, so you'd have a 1 and an 8 which is code 18.
There are two possible errors for a code 18:
1) Passenger seatbelt pretensioner circuit short to ground.
2) Passenger seatbelt pretensioner circuit short to battery or ignition.
So basically you have a short somewhere in the wiring to the passenger side seatbelt pretenioner it seems (unless there is a fault in the pretensioner itself or the SRS module of course). Be careful when playing with SRS stuff