I have to give it to ya... it worked on the first tryI had this problem too. Easy fix and Inhitbthis glitch 90% now.
1. Start session. Spawn anywhere except arena.
2. Teleport to the job next to the Arena. This is a good double check to make sure the person you intend to targeting mode glitch off of is in a different targeting mode.
3. Enter Garage workshop
4. Walk up to RC, and hit modify.
5. Count to 3. Normal 3 seconds, not fast or slow. Regardless is mechanic talks or not.
6. At 3, go to home screen and open YouTube.
7. Count to 5. Go back to home screen and click on GTA 5
8. You should be forced to single player. Rejoin an invite only session.
9. Teleport back to job next to Arena
10. Walk in arena workshop.
11. Enter donor vehicle. Modify.
A... If your car goes into the workshop next to mechanic . It failed. Restart glitch. (10% of the time)
B. If your car stays in it's parking spot, change one thing and sell. I usually change armor.
12. On infinite black screen, join friend in diff targeting mode. Accept first alert, decline second.
13. Hop out of donor vehicle, and go to receiving vehicle
14. Hit gas, and drive out. Mods should stick.
15. Jump out of car and watch it disappear.
16. Go back in arena and jump back in receiver vehicle.
17. Drive it into the mechanics bay and modify at least 1 thing. This saves the merge.