Why in my 1990 Honda Accord, the lights keep flickering on and off when parked and off.?

Could be so many things. You say lights, be more specific, dome lights, headlights, running lights, or combination. My best guess is the ignition switch is bad, when those go bad odd things can happen. Or could be the headlight switch is bad and shorting out. Move the interior dimming dial up and down to see if that works ok and is not causing a short. Might be a ground wire on the alternator or an electrical connection is loose. If you can not figure it out, this is best taken to a honda focused shop or the dealer. They will have a check list to work through to fix the problem. A lot of regular shops do not have that detailed check list, and they just guess, you pay for a fix that does not work. a>