JOMA

i have a cobra cb with the pa option so i hooked up a cheap walmart speaker amp up to a pa speaker from radio shack. everything works fine when the jeeps off but once i start the engine it doesnt work anymore. if i have the keys turned in the ON position but the engine isnt running it works perfect so i cant figure out why having the engine on makes it not work. any ideas?

thanks,
joma

ps. the amp positive wire is directly to the battery and the ground is wired to the engine ground bolt on the firewall