First, using a volt meter set for DC voltage, check the car's electrical system, battery, alternator electrical cables. With the car off the battery voltage needs to be around 12 volts. With the car running, the voltage should be between 13.8 to 14.8 volts. If not, your electrical system may not be charging properly and will need to be repaired.
The tuner power connection should have around the same amount of battery voltage. If the tuner has a big loss of battery power, then check for loose wires or connections. Also look for corrosion at the battery connections. If the connections at the battery do have corrosion, remove the cables and clean off the corrosion from the battery and cables.
The tuner needs to have constant power (+) and ground (-) with the key (car) on or off. If you lose your + or - with the key off, then you will have to check where the + or - is getting disconnected.
The tuner may also have a second switched + wire. The switched + will have power with the key on and no power with the key off, it will depend on the type of tuner you have. Check for proper voltage, power and grounds at the tuner's power connection when the key is off an on. If all does check ok, then it could be a bad tuner or radio.