Getting something, anything, outside the vehicle body usually helps tremendously.
A dedicated 700/800MHz antenna is a good choice, but since you'll likely want to listen to other stuff, might as well invest in an antenna that will let you get more out of your radio than just a single band antenna.
NMO mounts -are- the industry standard. Using an NMO magnetic mount base will open up the most amount of possibilities to you.
There are several brands out there, my favorite was always Larsen. Good product and the company has been around for a long time.
Here are some examples, you decide yourself where to purchase it:
https://www.tessco.com/products/displayProductInfo.do?sku=516684
https://www.amazon.com/Larsen-Magne...127&sr=8-2&keywords=larsen+nmo+magnetic+mount
As for the antenna, the Larsen NMO-150-450-800
Larsen Antennas
Good all around antenna that will cover the VHF High, UHF and 700-800MHz bands well.
A dedicated 800MHz antenna will work well. It'll work OK on 700MHz, also, since you are not transmitting:
https://www.amazon.com/Larsen-800-8...8-4-fkmr0&keywords=larsen+nmo+scanner+antenna
Won't work quite as well on VHF and UHF.
Don't expect either of these antennas to be good performers on VHF Low Band, CB, etc.