AM reception/antenna

[longo304]

Purchased a used 2011 tahoe this year with standard radio. Reception was very weak for AM, FM fine. I upgraded radio with screen, gps, etc. AM reception is still very poor. I attach a standard auto antenna to radio and stick it out the window and AM is great. So, existing antenna must be bad. Where is the antenna for AM????? Shark fin on roof?? Would greatly appreciate any help.