I have a 94 Elantra. The last couple of weeks when I drive the radio goes off and the "code" sign comes on. I re-enter the code and the radio works. Today I started it and the radio went off and there was no code sign. Then when I went to start it and I couldn't get it out of park. The button would not push in and the trunk opener button in the inside wouldn't work. After about six tries of starting and restarting I could finally press the button in and get out of park and the truck opener botton worked. Any one know what's gong on?

 

with the car not shifting out of park could be the brake switch or the shifter solenoid going bad. as for the rest of concerns, the radio could be a blown fuse, the trunk faulty/poor connection.

 

All three items could be independent problems surfacing all at once like madindian1 suggest, but I don't think fuses are involved since you can restore the radio without replacing a fuse (or at least you didn't mention fuses). The problems appear intermittent, so something is disconnecting/connecting or a a relay is sticking somewhere. I'm not aware of any relays in the radio power circuitry. I suspect all three items are related, the result of a common ground connection that is loose or going bad or a bad wire feeding voltage off the battery. Check if the car has one or more smaller leads hooked to the positive side of the battery. Most of us see the large main power lead off the battery, inspect it, and seldom take notice of the smaller ones. I'll bet you'll find one that is heavily corroded.

If you can't find such such a wire or none exist, then you'll have to diagnose the circuits one-by-one. Keep a multimeter handy in the car and see if you can find the power lead to the radio under the dash or the lock out solanoid. Next times one of these hickups, you can start checking voltage and tracing wires.

Good luck.