Drill a small hole at both ends of the crack. This will prevent it from getting larger. If the cap is made of fiberglass, it can be repaired. If it is plastic, don't know about repairing it. If you are handy, you can purchase a fiberglass repair kit and do it yourself. If not, most auto body shops can repair it (a lot of fiberglass parts are put on cars). Hope this helps.
2013 Ford F250 SD 6.7L
2014 Wildwood Heritage Glen 282RK
Certified Rumble Strip Tester