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Hello, you could leave your original wiring and add a seperate box like this one for your other new items. This would probabaly be the cheapest and easiest route to go.
The other option would be to get a new 21 circuit harness with box like this and spice in your existing circuits by cutting them off the old box and solder and heat shrinking them then using the remaining circuits and new wire for your power windows, door poppers etc. This would be a better solution for not much more money and less hours than rewiring the whole car.