Don’t assume all modern rigs, especially kits, already have polarity protection. They may NOT! I learned this the hard way when my little Ten-Tec 1340 QRP transceiver was mortally wounded because I reversed the power supply wires by mistake.
There are a number of methods that can be used to insure polarity protection. Some are more complicated than others, like a bridge circuit or a reed relay. Others simply depend upon properly mated connectors. Here’s a method that is both inexpensive and simple and can be used for most lower powered applications.
Buy a 1N4004 diode. Radio Shack sells them. Then you make a choice.
1. You can insert the diode in series with the 12V DC source. Connect the diode in the positive line with the cathode (the banded end) connected to the output side and the anode connected to the input side. Current only flows one way in a diode!
2. Connect the diode in parallel with the load. Connect the cathode (banded end) to the positive line and the anode to the negative line. Also connect an appropriate fuse in this positive line on the input side of the diode. Current flowing out of a reversed connection will blow the fuse.
If you choose option #1, connecting it in series, you will suffer a forward voltage drop. For the 1N4004, the specs tell us this can be as much as 1.7 volts. Do you have that much voltage to waste? If your answer is ‘no’ then all you have to do is use option #2, connect the diode across the load instead.
A simple precaution that can save your rig for 99 cents + tax!
Disclaimer — I cannot be responsible for your results; you modify your own equipment at your own risk!