Safety warning

[fishnjim]

That cigarette lighter socket in your classic wasn't designed to be an auxillary power adapter socket like we have today.   I'm old school so I still incorrectly call them cigarette lighter sockets, but the twain shall not meet.    Eg: the ones in my '58 are on a 20A circuit and are made to heat the coil hot enough to light tobacco.   That's enough to fry most electronics!   Accessory sockets vary but generally are in the 10-15A @ 12 VDC range.   Some are listed some not, but they all should be!
First, there's a plethora of power plugs coming from USA's "favorite Asian supplier" and there's no standards like UL listing or ANSI/SAE J563 used on them.   The problem is the lack of quality control and race to the bottom pricing on internet fed sales.   One has to be particularly wary of the largest internet based sales entity that plays fast and loose with pricing and who you're dealing with.   That bargain, might not be one, as it won't meet the intended use.   Even pricey doesn't guarantee it's well made and safe.   I won't go into how many items I had to throw out due to this because it makes me sick we'd let this happen again.   Recall the '60s import junk.
Right now, I haven't found anyone supplying these in any acceptable quality.   Why am I writing this, yesterday, had a near miss with a unit powered off such a plug.  I had it powered off a separate lead not a socket.
The power adapter plugs come in all sorts of flavors, but few state their rating or power, not most.  If it's charging a handheld device, it draw almost no amperage(microA) and not at issue here.   Those I speak are higher(>1A) amp uses.   I found discrepancies in that the wattage didn't match with the amp rating.  The one I had issues was supplied in a kit and came with an internal fuse.   The fuse didn't blow, as intended, the plastic melted and started to burn.   That's failure of a safety device.
The upshot, 1st, don't plug your accessory plugs in your classic cars cigarette socket and 2nd, beware that cheap plug you bought or was supplied in another product may be a fire hazard.   16 ga copper wire won't handle more than about 7.5A, but they routinely list them as 15A!   Make sure you have the proper wire size for what you're powering.   As length increases, so should wire diameter in DC circuits.   There are charts for this.

[TCheo]

I don't smoke, and everybody I know has a cell phone. I've replaced all the cigarette lighters in the car with USB outlets because charging is all they'll ever be used for. It's not original anymore, but it's practical. I didn't think of the safety factor either, good point.

[MaR]

The biggest issues with lighter sockets in old cars is rust. It's rare to find one that is not rusted to some extent and rust equals resistance and resistance equals heat. That's OK on a lighter but on a plastic plug it's a no-no. Also, 16 awg wire is just fine for a 7.5 amp load or even a 15 amp load at 12 volts even with cheap vinyl insulation.

[Cape Cod Fleetwood]

EXCELLENT post... 'Some' of us already understand this but its important for the masses.
Many elderly vehicles don't want any part of halogen or LED headlights or LED interior lighting either,
they weren't designed for it. I've seen HVA spend a lot of time, and customers spending a lot of money,
to have their classics correctly (and safely) updated for modern electronics. If you don't have a working
knowledge of elderly car electrical and the proper process to upgrade for modern electronics, pay a shop
that specializes in classics to do it for you. It'll be cheaper than seeing your car in flames.

[cadillacmike68]

DC Electricity 101 here.

The lighter sockets, old and new, have a positive and negative connection which is OPEN until something is put in it. They run off Alternator output which is anywhere from 12V DC to 14.3V DC 20A max or whatever the body fuse is rated for.

When you plug a lighter element in, the current passes through the coils on the lighter and then to the negative feed to ground. It's basically a resisted short and when the lighter element gets hot enough from this it pops up thereby re-opening the circuit.

The socket only gets hot from thermal transfer NOT from electrical current transfer.

When you plug a power adapter for a phone, etc. in, it Only draws the current needed by whatever you plug into the adapter (up to the max rating of the adapter). It is NOT going to overheat UNLESS you over draw the adapter's capacity.

Now, If you plug a 2 amp draw phone into a 1 amp rated adapter, you WILL have problems, so get an adapter rated at 2 amps or better.

I use a 2A rated adapter in my 1968 DVC with no problems.

Most phones sold in the last 7-8 years require a 2A or better power adapter. Get a power adapter from the cell phone stores if you are unsure.