I just went though this a couple of months ago on my 2000. There is a data test port in the engine compartment right near the top of the intake manifold (about a 1X2" black box. It has a removable cover. In the service manual under the supplemental restraint system it will show you where to install a jumper in this test port to set off the diagnostic codes for the Air bag warning light. After you install the jumper, the air bag warning light will blink a numerical code (you count the flashes) and that should highlight the area of the system that is causing the issue. I am guessing the '99 has the exact one or something similar.

Mine issue was the passenger side restraint (i.e. Seat belt). On line videos show how to use a multimeter (measuring resistance) to test the seat belt squib. Mine tested bad. Rather than the dealer recommended replacement ($400),I sent my seat belt asssembly off to a company in Atlanta that rebuilds them (about $100). Fixed my problem.

Like you, I initially suspected the clock spring because that seems to be common.... but I would suggest you read the codes first.

The system is actually very simple.... other than the air bags and seat belt restraint parts themselves is a SRS computer just below the radio and the two sensors located in the engine bay that sets the system off. And anything with yellow wiring should warn you that it is part of the SRS and to diconnect the battery and wait before doing any work on it. The Toyota manual advises not using a meter to measure for continuity across the squibs..... but the on line videos show that it is effective without setting the devices off. The connectors connecting the air bags and seat belt squibs are "special". They have a little shorting bar that shorts the contacts when you pull it off..... I assume this is to keep from having any stray voltage or currents from setting it off. To test these devices you will have to use something like a toothpick or non-conductive pointy ended thing to hold the shorting bar open while you test for continuity.