if it is entering into the password menu when you turn the truck on i would check the control handle buttons and make sure none of them are stuck. It seems unlikely though as you have to press horn and right shift buttons simultaneously to make it enter into diagnostic menu mode.
I'm not aware of these trucks having any sort of password access mode so there is definatly something strange going on to cause it to do that.

one thought is to check the ks1 relay on the TMM and make sure its not faulty. Another thought would be the relay driver board on the front left under the top cover. Those were notorious to cause issues if corroded up.

I still tend to lean toward something being wrong in the control handle though. You might want to pull it apart and make sure none of the wires running through it are not pinched. If anyone has had it apart at any time they are very hard to reassemble without pinching wires as they have to run through it a specific way and if they dont you get pinched wires.

btw the default password and superword for these cat trucks are cat1 and cat2 , unless they have been changed. You can try and enter those and see what happens. If there are any issues in the handle you will know right away when you try to enter the digits if it does not want to scroll through them correctly and accept them.

just some thoughts on what to check out at first