Does the control unit already exist in my car if it came equipped with rear parking sensors? In other words, can I buy aftermarket front sensors and connect them to the control unit (if it already exists?) and they would appear in the optical parking screen? Sorry for sounding like a newbie, don't know how control units work. Thank you.
You would need to find out if SEAT fitted different part numbers of parking sensor controller to cars with rear only or rear+front parking sensors, this should be easy by searching the online parts listings. If there are different versions of parking sensor modules fitted, and not just different coding on a similar module, ie the rear only module being a 4 channel one and the rear+front being an 8 channel one, then you would need to replace that module or larger controller that covered that function, or if all the modules were the same you would only need to recode it. Maybe scan the car to see what the part number of the parking sensor control module or sub module of another component that covers this task so that you know how things are at the moment.
To make life simple, I'd opt for buying a set of 4 VW Group sensors, getting a proper upgrading place to cut/punch holes in the bumper skin for you, unless you buy a correct tool - the plan is always to cut/punch through from the outside so that the outer surface edges get rolled over and inwards otherwise it will look crap! Next you would need to buy or manufacture a front end cable form that terminates in a 6-way connector so that the front end can still be removed for maintenance etc. Then another cable form to connect this first one to the car's central controller, new buzzer, maybe a switch and the rear panel located parking sensor controller - or other controller that covers this function. Then code these changes into the car.
Edit:- it looks like the parking controller is mounted near the front, which is handy for adding in extra wiring, there is a different part number for the "rear only" and "rear and front" cars, so I'd think a 4 channel and a 8 channel one. The front sensors listed need to be "right angled connector" versions, also a sealing trim located over each one and mounting brackets bonded to the front bumper covering using structural adhesive - unless you wanted to need to refix them every few years when the sticky tape unstuck!