Any FM antenna under metal is not going to be very effective. AM will be a complete loss, it would be like being under a bridge all the time. If you can hide it under glass/plastic it may be more effective though. Under the package tray would work, that may work better than the factory windshield actually.
If the car has/had a working radio chances are there's an antenna in the windshield (if there's no hole in the fender).
By the way, I got new front windshield for my car for $180 installed in my garage. It's a lot of money but if it's old glass and/or if you want to repaint your dash while the glass is out, that's an inexpensive upgrade. If your car wasn't wired for a windshield antenna, you will still have a problem getting the antenna routed into your car. So if your car doesn't have an antenna windshield, check the outside bottom center of your windshield for a lead, should be on/under your cowl to plug a windshield antenna into. If no lead, that Crutchfield bar is your best option.