VW Electrics Smooth 12v/5v supply
Covkid & Ghost123UK:
Simple Low Voltage Power Supply
These days, we run items that need a lower supply than 12.5V, and if you've got something that uses say 9v or 5v, although you could plug in an adapter, this isn't always ideal if the item is permanent. You can get round this by making up a simple voltage dropper - for that you'll need a voltage regulator.
Relatively cheap, voltage regulators have three legs and in various ratings (see last two numbers) including but not limited to:
Unfortunately the LM7812 can't be used in a T25 as it needs a headroom voltage of about 14.5v (see below for a 12v solution). However, with the addition of a few capacitors to remove interference from other electrical items, you can build a simple and effective power source at lower voltages. They're cheap and easy to build and a whole lot less than ones you can buy - even on ebay. Check the amp rating will be suitable for your appliance though.
Heres a simple circuit using an LM7812 regulator (for reference only):
'Input' refers to red wire from battery or other dash 12v power lead, 'output' to the item that needs the fixed supply. Earth (chassis) is common to both.
A great video explaining the principles can be found here: https://www.youtube.com/watch?v=GSzVs7_aW-Y
12v Power Supply
Mains Hookup Power Supply
If you really want to go to town on this, you can create a really good, stable little power supply on hookup (240v) from an old computer ATX power supply as follows: