If this is going onto a propane truck, height will be you enemy up top. You want an antenna that is as close to 108" total in length. Don't believe any of the hype about 5/8 wave 4ft antennas. Unless that antenna is near 23ft long, it's not s true 5/8 wave. Look for a good 1/4 wave type antenna. The Wilson trucker antennas, or even the Sirio trucker performer 5000. Use a bottom shaft long enough to get your loading coil above the roof line. This may take some measuring and may be more or less than 10". Mount the antenna to the mirror mount as long as it's metal. Then make sure it's bonded well to the vehicles door and then frame. Use 1/2" or larger flat ground braid strap. And cover it with heat shrink. Also make sure you have a good ground as well. And you don't have to run both the POS and NEG wires to the battery. Run the positive wire to the battery and then find a frame ground bolt to attach the ground to. Under your dash or your seat bolt, will work. And use
NO-OX special "A" on all connections. It looks like a brown grease almost. Here is a pic of some. eBay sells it for about $8. For a small can of it. Also don't worry too much about the amp at this point. Get the antenna setup as good as it can be!! Also good coax and connectors are a must. I use LMR240, and amphenol connectors for my mobile installs. What you use is up to you.
As far as companies that make good coax goes, Belden, Andrew, Commscope, Times Microwave, Eupen, Shireen, Davis RF Corp., and a few others I am sure I missed. These are good companies that make good coax. Don't skimp on your antenna setup, that is all I will say about that. It will make or break your station.
Now as far as the 989 goes, leave it stock or have someone that is truly qualified turn the power down a little. Like a 3 watt Dead Key and 8-12 watts pep. Same on SSB. Keep the power at stock or less power. This will save the final! Also you can add a heat sink or remove the internal speaker if you use an external one and this does help some with releasing the heat.
The wiring is pretty straight forward. Red to POS, black to Ground. If the negative wire isn't fused, don't connect it to the battery. JMO. I have installed quite a few radios aka never had noise issues by going to a frame bolt and keeping the ground wire as short as possible. But everyone has different opinions on this subject, so I would do your own research and see what will work best for your install as all are different. As far as mounts go, get a good one. Breedlove makes some very nice mounts, as well as some other companies. Stay away from the truck stop junk though. Again, JMO.!
Yes I do own a uniden 980. Have since they 1st came out. As well as the Wireless Mic.
You can get the radios off Amazon for a little over $100 new shipped to your door.' The mics have come down a lot in price as well. Costing about $50 new shipped to your door. And the wireless mics do indeed sound better than the stock ones do. Again, JMO. I have one and the only mod I performed was to drill out the moc hole with a 5/16" drill bit by hand. Then I used a 3/8" drill bit to add a larger taper without going all the way through the mic. And it works just fine!! Gets good audio reports, and it's wireless, with the base of the unit plugging into the radio and the mic being able to get about 75ft or so away from the radio and still be able to be used. The speaker in the wireless mic isn't the best, but you can understand what everyone is saying just fine!! Love it when I walk away from my vehicle and keep talking on the radio and the looks you get. Had to show my local pal by going into his house and then transmitting. He couldn't believe the mic worked like it did. At any rate, for the money, you won't find a better new radio. They are dead on frequency out of the box and don't need much if anything to operate well. With a Sirio performer 5000, A Breedlove 3" one hole mount, on top of an E-350 ford work van, I am able to reach back to base stations about 30 miles away when the band isn't roaring with DX. This is on SSB, and my power turned down to about 8 watts pep. As I said earlier every install is different and your mileage may vary. But if you get a good antenna and install it correctly, you have the possibility of getting out to 20+ miles depending on the terrain and other obstructions. Good radios. Great radios for the money. And the mic is a good buy as well. These are all JMHO's, and from my own experience using the Uniden 980. Hope some of this helps, and the other posts above mine are have lots of great info as well!! Be safe and take your time!! Have a good one.