How do you change the light on these bridges? They are missing the flashing device.

A part # or close up might help.  You can't even tell if it has incandescent or LED lights.  How to flash them depends on knowing.

When you post for help, include the kitchen sink

There is a small 12 volt AC,  PC, (Printed Circuit), board, that is designed to fit in one of the bridge support piers.   For some reason the bridge had three piers.  Middle pier was always a sore discussion, here on the forum.   

Bridge with two deck bridge extensions. 

The bulb is a screw base (midget or T1-3/4 IIRC).  I use 18v bulbs dipped in red Tamaya transparent paint.  Power (flashing) is supplied via a circuit board on one to the piers.  Use a variable A/C power source to get the brightness you want.

To test the lamp (do this first), remove (unscrew) and jump in some power OR use an ohm meter.  Bulbs will read 0 if bad, any obvious reading means its good.

If the board is there, grab a close up, and note component #s if they don't focus.

Making a flasher is pretty easy and there are likely premade Ebay boards dirt cheap too.

Both sides of board if you can 😉

This is the board that came with the MTH bridge.  Mounted under the layout, close to the bridge.  It can be turned on/off, with the Cab1 remote, and original Command base, as wired.   Flashing light.     Picture of other side of board, a bit more difficult. .    One board, one light.  I don't know if one board would work two lights.  Each or your bridges should have the board pictured.