If you have a spare car stereo deck you can sell it or make a stereo for your garage. A project like this has many options. You can construct custom deck enclosures, amplifiers, custom speaker enclosures and power supplies with enclosures. It's best to start with the basics and upgrade as desire and motivation dictate.
Brush up on crimping and soldering techniques. Remember that every connection must be covered with heat-shrink tubing or electrical tape.
Obtain a wiring diagram for your stereo deck. Different decks use different colours for their power needs. The most important thing is to know which wires need positive power and which are ground (negative). Carefully label each wire with tape and a description. Generally, decks have two wires to receive +12 volts for power and one for ground (negative). One positive wire supplies constant power to the unit (to maintain the clock and station presets), and the other wire only supplies power to operate the stereo when the car is turned on. Strip about 1/2 inch of insulation from each of these wires. Twist the ends together of the two positive wires and label them as "+12V". Strip about 1/2 inch of insulation off of the negative ground wire.
Determine what you will use as a power source. You can use any 12-volt automotive or marine battery. The positive terminal of the battery needs to be connected to the wire pair labelled "+12V". You can use solder or crimping connectors, but make sure you insulate them with either heat shrink tubing or electrical tape. The negative terminal of the battery needs to be connected to the wire labelled "ground". The battery will eventually need charging, which can be accomplished using a battery charger.
An alternative to using a battery for a power supply is to use a 12-volt source that can be plugged into a wall outlet for constant power. An old computer power supply will work. Use a voltmeter to determine which wires coming from the power supply are the 12-volt positive and negative wires. Splice the power supply wires to their appropriate stereo power wires. Also, your local electronics store sells a 12-volt power supply that can be used to provide power for the stereo.
Connect wires to a set of speakers. The wiring diagram will tell you which wires coming out of the deck go to which speakers (left and right, and possibly front and rear). Attach speaker cable to each of the speaker wires coming from the deck using solder or crimp connectors, and attach the other ends to the speakers. Use heat shrink tubing or electrical tape to insulate any exposed wiring.
Plug an antenna into stereo's antenna jack if your unit has a radio.
If you so desire, you can now build an enclosure for the stereo and power supply.
Test the unit in a car first to make sure it works.
Be very careful not to short anything across the battery terminals.