Your next issue will be installation gear for you new radio.
As I installed a 2DIN radio I will use my experiences as an example. Many of the factors in this example will affect most others installing a new 1 or 2DIN aftermarket.
If you have B&ME you will need to relocate the module (which was the lower half of the original radio , with that large blue plug) to another location else the mileage on the drivers display will flash at you all the time (and trust me it gets annoying).
This is achieved with a cable that I got from a Pioneer retailer called 'CTBLUE.MELEA' which is actually produced by Connects2 and rebadged by Pioneer. I am sure other manufacturers has the same 'deal'. This is basically an extension lead for the B&ME cable so you can put the unit elsewhere.
The 'best' place seems to be behind and below the glovebox and this requires you to remove the glovebox.
This is done by removing the panel on the side of the glovebox (usually covered by the passenger door when closed) which normally will have a keyhole in it to disable the passenger airbags. This panel just pulls off and the plug for the keyswitch can be removed.
Once it is out , you will see a screw at the end of the silver (in my case) strip at the top of the glovebox. Undo this screw and the strip will pull off (it's held in along the rest of its length with pins in grippers , it's tight but just pulls off) , dont just pull from 1 end as the leverage will break it , pull at one spot and then move along as you progress.
Undo 2 large plastic 'nuts' from the underside of the glove box that hold the front edge of the 'felt' under panel to the glovebox in place and then remove the underpanel.
Open the door of the glovebox and undo all the screws that you can see around the edge of the glovebox and under the strip you removed. The glovebox will just pull out when you have found and undone them all. The will be 2 plugs , one for the light and 1 for the USB (if you have it).
Route the existing cable from the center fascia area back and up and you should find a route to the glovebox area. You will want to get the cable to go this way far enough to see the plug in the glovebox area. This route will be the same route you might use for a new handfree mic or a gps antenna so if you need these , route them through now. I went behind the A-piller trim for my GPS antenna and it works well there. A new handsfree mic can then be routed behind the same A-piller trim and then further up to the roof courtesy light area.
Once you can see and access the plug from the glovebox area , use the CTBLUE cable to extend it further and route this to the back of the area the glove box was in , aim for the lefthand side if you have a RHD car and go for the very back of the empty space that the glovebox was in.
Refit the glovebox , testing that you can reach the extended B&ME cable from under the glovebox before you screw it back up. You are going to reconnect the B&ME module and 'jam it up behind the glovebox and use the underpanel to hold it in place.
Don't forget to reconnect you plugs , the USB one , the glovebox light one and most important (as it will cause an airbag error) the one for the key that disables the passenger airbags.