My 2003 ford expo has a constant airbag light lit after starting the veh. I have a new sensor, can't figure out were 2put it.. Please help.

Here's a quick check list for most common cost of SRS / Air bag problem.

Dirty contacts on driver and passenger seat belt buckles

Dirty contacts on seat belt connectors (under seats)

Dirty contacts on driver and passenger sides weight sensors (under seat)

Seat covers

Weight on passenger seat
(Example: grocery package trigger SRS / Air Bag light on the way home.)

I recommend you to pass an vacuum over the contacts and use CRC electronic parts cleaner to rule out connectivity problem.

I DO NOT recommend disconnecting the negative battery connector to clean the code(s).

Without the code(s) the dealer will takes "BILLABLE" time to reproduce the problem.

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Cheers,

Mustgo

First, you need to read the Lamp Fault Code, LFC from the flashing light. Here's how:

Lamp Fault Codes
When the restraints control module (RCM) detects a system fault, it will cause the air bag indicator to flash a coded sequence: a lamp fault code (LFC). The code is two digits. The first digit is flashed with a 0.5 second interval between pulses. There is a 2 second pause before the second digit is flashed, which also has a 0.5 second interval between pulses. There is a 5 second pause between each display of an LFC.
Each LFC is flashed five times after which the air bag indictor will remain lit for the remainder of the key-on cycle. If there are multiple LFCs, each LFC will flash in order of priority.
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The fault could be in the air bag circuits, the canopy circuits, the pretensioner circuit on the seat belts, etc.
Report back with code.

here's some of system.

SECTION 501-20B: Supplemental Restraint System 2003 Expedition/Navigator Workshop Manual
DESCRIPTION AND OPERATION Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS) The air bag supplemental restraint system (SRS) is designed to provide increased collision protection for front seat and second row outboard occupants in addition to that provided by the three-point safety belt system. Safety belt use is necessary to obtain the best occupant protection and to receive the full advantage of the SRS.
This vehicle line contains dual stage deployment (advanced restraint system) driver and front passenger air bag modules. These vehicles can also be equipped with optional safety canopies that deploy from the A-pillar to the C-pillar upon a side impact or if a rollover condition is detected. If equipped with safety canopies, a unique restraints control module (RCM) is used.
See the following illustration for deployable component location.
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS) Deployable Components Item Part Number Description 1 611D10 Passenger air bag module 2 042D94 Passenger side safety canopy module 3 042D95 Driver side safety canopy module 4 61203 Driver safety belt buckle and pretensioner 5 043B13 Driver air bag module 6 14A664 Clockspring 7 61202 Passenger safety belt buckle and pretensioner
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Item Part Number Description 1 14B321 Restraints control module (RCM) 2 14B345 Second row, C-pillar side impact sensor 3 14B345 Second row, C-pillar side impact sensor 4 14B416 Driver seat track position sensor 5 14B345 Front row, driver door, side impact sensor 6 14B006 Front impact severity sensor 7 14B345 Front row, passenger door, side impact sensor