Re-Installing the Factory Head Unit

[The Blue IS200 TTE]

Hi Guys, new member to the forum here and was looking for some help. 
 

I had an aftermarket head unit fitted to my Lexus back in March and for some reason, the right side sound only works. I spoke to the installer who said he thinks the wiring needs to be cut and rewired but I haven’t been able to get it done by him as he has been busy with other work. 
 

Now, I’ve gotten to the stage where I’ve just had enough and want to re-install the old head unit but the problem is, I’m not very good with opening up the compartments and knowing what goes where. 
 

Does anyone here have a guide of any type to help me with it? Any information such as what tools I need etc and what to do with the bypass lead and amp lead would be helpful. 
 

My model is the satnav version if that makes a difference. 
 

Thanks