Yellow warning light with key on it?

Hi please help, my saxo 1.5d is having problems when I go to start the car the yellow warning light comes on and starts flashing also a dim siren noise comes from a little black box attached to the water reservoir, My speedo goes up to 50/60, and my miles start running as tho im driving. I have taken this little black box off and disconnected the cables into it, This does not work. On the advice from someone I removed the battery and cleaned the earth wires that lead from there this also didn't work. I have managed to get it in to a garage and he said he's going to put it on to the diagnostic machine and let me know the results my worry is i don't want to be ripped off im not saying he will but i want to be careful, i have read some other questions on here and someone mentioned about an aerial you put into your barrel, where would you buy this from and how would you put it the barrel?? and what does it do. Thanks