I lost all the programming and I have piece together how to do almost everything except get line 1 to light up when in use on another phone.

I believe this is by default assigned to the top left key on the phone however I assigned as an extension instead

I can assign a key to access the trunk however not light up when others are using line 1.

Any ideas on how this can be done?

Thanks.

Is this for that 124i that you've been working on? Or are you working on some other model of system?

Assuming this is an I series phone, the appearance of the line has to be on the phone in order to lamp. If you replaced it with an extension, then no, it will not lamp.