Need help. Step rail intall. Internal nut broke loose

I am hoping to get some advice. Installing the brackets for step rails on my 2016 Tacoma and run into a snag. On the front passenger side one of the internal nuts broke loose when I was tightening. There were no plastic plugs in the holes and the weld had probably got some rust in it and it broke loose. Cut the bolt off. There is a rubber plug about 4 inches from the front bolt holes but not enough it would be pretty hard to get a nut and lock washer in there and even if I could I couldn't get a wrench on it while tightening the bolt. If I could fish a bolt down though the hole with a star lock washer that would work but I have no idea how to do it.
Any one have any bright ideas I would appreciate it. Thanks

 

Riv-nut.

 

Wow. Quick response hiPSI. No familiar with Riv-Nut. I will look it up. thanks

 

More readily available solution, get a long bolt of the correct thread, get a nut that fits, cut the bolt head off, thread the bolt into the hole, thread nut onto bolt, hold the bolt with pliers/vice grips/channel locks, then tighten the nut while still holding the bolt

 

You may have to enlarge that other access hole in order to get a loose nut in the location of where the welded nut should be.

 

Tough one.
- Sounds like a hint for a proper slider purchase…