I've installed numerous antennas on these series GM pickups. I've still got a 2007 at work that we use.
A magnetic mount will work, but you'll need a lot of coax to go down the back of the cab, under the truck, back up to the firewall and on to the radio.
Not ideal, and in the long run the magnet mount is going to damage your paint, and the flapping coax will too.
If roof top/hole drill is absolutely not an option, then go with a bracket mount off the hood. It's going to suffer from hampered performance, but it'll get you something.
Ideally, the NMO mount in the center of the cab roof is the ideal location. That gets the antenna up in the clear and provides an ideal ground plane under the antenna.
Plus, it's really easy to install in these trucks. Center of the roof, if you have a crew cab or extended cab is easy to do by removing the dome light. Never done a regular cab version, but routing the coax above the headliner should be simple.
Even going back above the third brake light will be easy. That requires removing the 3rd brake light, drilling the hole, and routing the coax down the back of the cab. Doesn't get any simpler.
No matter what you chose, NMO mounts are the industry standard.
Avoid the Chinese/no-name brands. Might save you a dollar or two, but you'll pay in the long run.
Here's what I'd strongly recommend:
This mount:
https://www.theantennafarm.com/catalog/laird-technologies-mb8umi-1131 Already has the "mini-UHF" connector installed that your CDM1250 uses.
This antenna:
https://www.theantennafarm.com/catalog/laird-technologies-qw152-720
The nice thing about 1/4 wave antennas, like I linked to above, is that they are very broad banded and will work well across your frequency range without issue. These pre-cut ones are usually pretty close to right on and will work well for you. Plus, 18 inches tall, so unless your truck is lifted, you won't have any problems with clearing garages.
If drilling the hole is out, then I'd recommend:
This antenna:
https://www.theantennafarm.com/catalog/laird-technologies-b1442ns-7116
This is a half wave VHF antenna that does not require a ground plane to work. Since you'll be mounting it off the fender, the ground plane will be wonky since it's on the fender. It's a taller antenna, and some of it will stick up above the cab.
This bracket mount:
https://www.theantennafarm.com/catalog/laird-technologies-lbt3400-3843
This will screw into the side of the fender between the hood and fender.
And this mount:
https://www.theantennafarm.com/catalog/larsen-nmokhfudfme-1126
This mount has a different method of getting the cable into the base from the one above. Since the underside of the NMO won't be inside the warm/dry vehicle, you need to make sure it's protected from rain, road salts, etc. Ideally you want to slide some marine grade heat shrink tubing over the cable and where it enters the base. Heat it up so it seals that point well. The other style mounts will allow water/salt/oils to get into the mount and coax, and will cause issues with corrosion.
That mount also has a standard FME style connector on the end. The connector is small enough that you can route it through the firewall and install this adapter on the end:
https://www.theantennafarm.com/catalog/rfi-rfe-6105-1216
Ideally, you want to cut the coax to the shortest length necessary to reach your radio, plus a few inches for making repairs. Preterminated cable is not the best choice, but unless you have the tools/skill or are willing to pay someone to install the connector for you, it's a better solution in most cases.
I'd steer away from the mag mounts if you can. You can look at mag mounts here:
https://www.theantennafarm.com/cata...unts-265/nmo-magnet-mounts-with-mini-uhf-274/
Problem is, the coax will probably be too short unless you get really creative with the install.
