Since cars can travel into fringe areas outside of major cities, dedicated car receivers generally have a RF amplifier stage before the intermediate frequency IF stage. The RF stage amplifies the incoming signal before it's converted in the IF stage. In other words, the RF stage 'boosts' the incoming signal.
Radios like the Tecsun 680 don't have this extra stage. The signal from the antenna goes right into the IF stage. (This is called direct conversion). Direct conversion radios generally don't have the sensitivity of ones with one or more RF stages before the IF.
You can add this functionality to a direct conversion receiver by adding a tunable outboard RF amp. These are known as preselectors. The amplification they provide increases the signal and the sharper tuning reduces interference from adjacent frequencies and it helps eliminate images.
I'm not endorsing MFJ, but here is a link to one preselector they offer: MFJ Enterprises Inc.
If you look around you'll find them made by other companies.