Now for the not fun part. Working under the dash can be nasty.
In the images I posted, you will see one end of the Lockup Solenoid coil is grounded and the other end goes out on a Lavender colored wire to Socket T4 Pin 13.
Plug B11 plugs into T4 and on Pin 13 the wire color changes to Red-yellow stripe then goes through the firewall to Plug B54 Pin 16 that connects to the TCU.
T4 and B11 are located on top of the bellhousing. It is a 20 pin connector.
The Harness diagram shows the TCU is located around the steering columb. There are 2 plugs in it, B54 and B55. I have seen older units that had TCU right on the label.
You will tap into the Red-yellow stripe wire under the dash or under the hood with one of the bulb wires and the other wire on the bulb will go to ground.
The bulb will light when when the TCU turns on the Lockup Solenoid and will go out when it is released.
If the bulb goes out when you brake but it is still in lockup then the problem is in the tranny.
If the bulb stays on when you brake then It is the TCU or maybe a sensor input.
I know that most of the valves in the tranny have springs to return the valve piston when the solenoid releases. If the spring is weak or broken or there is sludg build up the piston will not move properly. One of the images is the valve and spring.
The Lockup Solenoid valve routes pressure between two ports going to the torque converter - Lock-up apply pressure and Lock-up release pressure port. So, if the problem ends up being in the tranny, it could be the solenoid, valve, valve spring, a leak in the release port or the torque converter it's self.
We'll see what the light says.
You can save or print any or all of this info. I cropped and resized the images to make the file size smaller.