I've put several ham radios in various vehicles I've owned. As soon as detachable faceplate radios came out that's all I've used.
Adding to N7VRZ's wonderful comments, I guess I'll throw my 2 cents in from KM2U.
First of all, here is a link to my installation on my Toyota Tacoma, yes, it's not a Mercedes, but it some may find it helpful anyway.
Trying to install a 2 Meter only radio could probably be done on your budget of $300. If it were me, I would use either a lip mounted Comet antenna, or possibly fabricate a Wire J Pole antenna. I've seen these made from TV Antenna wire. See: http://www.work-sat.com/Antennas_files/FONG-DBJ2.pdf
If you could up your budget a little, I would buy a used Icom 706MKIIG radio and a
Tarheel Antenna. See: http://www.tarheelantennas.com/baby_tarheel
This would allow you to remotely mount the radio in the trunk and control it using the radio head loosely mounted in the vehicle. You could use something like a fabricated bean-bag to lay between the seats with the control head velcro-ed to it. You would have 2 Meter op's as well as the full HF Band with the exception of 80 and 160 Meters. Trust me . . . when you make that first long distance contact, you'll be hooked on HF, but you'll still have 2 meters as well for local repeater Op's. If you go this route, you will need to add two things in addition to the
Tarheel antenna. You will still need a 2 Meter Lip Mount Antenna and you will want to add several bonding straps to your vehicle as I did to my Toyota Truck. These bonding straps are necessary when operating Mobile in the HF Bands, but are NOT NEEDED for 2 Meter only Op's. The reason is, when operating in the HF Bands, you need to "Capacitively Couple" your vehicle to the ground it travels over for your antenna to radiate. Picture this schematic in your mind to help you understand this. Draw on a piece of paper a small square and label it car. Then draw the symbol for a capacitor and connect it to the car. Then draw the symbol for a ground and connect it to the other end of the capacitor. That in a nutshell is what I mean when I say Capacitive Coupling, it's as though you are dragging a capacitor from your car to the ground you are travelling over.
In 2 Meter Op's, your vehicle is sufficient to act as a ground plane to radiate the signal, this is not true at HF Frequencies. So, by using bonding straps as I've done in the video, it allows all your body components (doors, hoods, etc.) to become "One" increasing the area for the RF Signal to reflect from. Your entire car then capacitively couples with the ground and your RF efficiently exits the tip of your antenna.
Because this capacitive coupling occurs and is dependant upon what surface you are travelling over, you will sometimes need to retune your antenna to maintain a low SWR ie; such as when driving over pavement then turning on to a dirt road. Yes . . . I know your question was in regard to just a 2 meter radio install, but I thought I would suggest a world to you within Amateur Radio that might provide sooooooooooooooo much more enjoyment as well as motivate you to upgrade to a General
I should mention, I've worked Australia from my Mobile setup. Fell free to hit me up with any questions you may have and please enjoy the video.
That's Kink Mike 2 You