Right turn signal doesn't blink and tailgate lights don't work

[moonline]

Hi, I recently acquired a 1988 Chevrolet Celebrity, 2.8L, 255,000km.

About a month ago, my friend who previously owned the car tried to replace the car's radio by one that includes a CD player.
It seems that he tampered with the wires and the installation was surely unprofessional. It was almost impossible to add a newer radio to such an old car. Although the new CD radio is functionable, the installation caused some problems with the car's lights.
The left-turn signal does not blink and is just constant (both the dashboard arrow and the actual amber light do not flash.
The right blinker is perfectly fine. Additionally, 2 days ago, the tailgate lights on the back stopped functioning, what prevents me from driving after 5PM. The back breaking lights function well when I break, but when I turn the lights on, the back tailgate lights do not ignite. Again, the front lights work fine as well, although I noticed a slight change in intensity.

I did some googling and found that it might be a "blown fuse" problem. My friend told me that he checked the fuses beneath the steering wheel 2 weeks ago and they were allright.
I do not want to spend money on a mechanic, since I will have to invest into winter tires pretty soon.

Help would be greatly appreciated.
Thanks.