Many radio amateurs can pursue from home only very limited their hobby. This may be due to the rented apartment without antenna permission or the interference fog, which makes any reception impossible. Or you want to try something completely different. Then a car is a quite interesting basis. However, solutions like the one shown here are only permitted away from public traffic areas!
Power supply via the trailer coupling
A trailer hitch is quite a helpful accessory: Somehow beer benches, tents and other equipment have to be brought to the fieldday 
The ball also includes a socket for the lights on the trailer. But there you can also find a connection to which the battery voltage is constantly applied – and that with a connection cable that with 2.5 mm2 is much more reasonable than what I know from the sockets in the car. But according to Wikipedia it is not sure that this permanent plus line is always connected.
The picture on the right shows how the connection can be solved. I would come up with the idea to use a ready-made connection cable to slaughter, because that will be more stable and above all more watertight than any self-made construction. For the connection of my transceivers I use 2x 4 mm2 speaker cables, z.B. Pollin order number 560959. 25 m of it costs 30 EUR. Then the topic is ticked off for the next few years. 25 m of it costs 30 EUR and then the topic is for the next years for now checked off. You can still get this cable into the usual 12V plugs.
At this point, you should also consider electrical fuses: Of course, the connections for the trailer are fused as well. However, it is essential to pay attention to two things:
1. Each line must withstand the tripping current of the fuse in front of it. 2. The whole circuit must be so low impedance that the fuse can also blow in the event of a short circuit. If the outlet is wired with 2.5 mm2, you are on the safe side for the first criterion with the same cross-section behind the outlet. Depending on the length of the line, however, this does not have to apply according to the second criterion: Let's assume that the engine is off and the circuit is fused at 25 A. Then the entire circuit should have an internal resistance of less than 10 V / 25 A = 0.4 Ω. That's not much! Therefore I work there with 4 mm2. DL9NDF has given his adapter cable a 15 A fuse.
One more remark to the connection cables, which are usually enclosed to the mobile transceivers: The positive and negative wires are fused because the manufacturers require the transceivers to be connected directly to the battery terminals. The fuse in the negative line is necessary in case the normal ground strap comes loose. Because otherwise the entire battery current flows via the transceiver to the antenna base point. If one closes the transceiver however, like here or also my installation, on the other side of the ground strap on, the problem just described cannot occur.
The requirement to connect the transceiver directly to the battery terminals is, in my opinion, one of those old braids that I cut off during my installation: In the days of DC alternators with their mechanical switch (commutator) and mechanical charge regulator, many a electrical dirt Get out. Since it was quite reasonable to consider the battery as Filter to use. With today's three-phase alternators, this is virtually impossible. Otherwise, my 1-F capacitor performs this function at least as well.
The trailer hitch as an antenna base
In public areas, the practice described here is taboo, but on one's own weekend property, things look different: Nowhere else at the passenger car one has such a stable point. One should always make sure that this point is quite close to the plate. An extension coil here will create powerful vortex currents in the sheet metal and the spotlight directly above it will be quite clear capacitively short-circuited to ground. DL9NDF's solution avoids these problems by using a λ/2 radiator and letting it start a bit higher. A 10 m mast is held by the trailer hitch in good weather even without guy wires. One saves all effort with radials or other counterweights. What else such an antenna on Counterbalance needs, the car can deliver.
For stand-mobile operation, this is certainly one of the better solution options. By the way, here a quick release for a bike rack from Eufab was used. At this point, by the way, two requirements contradict each other: For trailer operation, the ball must be greased. For fixing with bicycle carrier or antenna the lubrication is rather hindering.