I don't know why they said that because I got mine installed by Suzuki dealership themselves. It might be because of close relations with the owner of the dealership but still warranty wasn't void then.
Yes they wont pay much attention to check if there has been electrical modification, if Suzuki people refuse to do it, get it done by an electrician. And yes please do be VERY careful in case of the bumper. It has a lot of clips and screws. Make sure all are removed before the installer tries to pull apart the bumper.
For reference check this link, they installed it themselves and charged me only 500 for installing that (labor charges for installing horn and removing bumper)..says on the invoice.
https://www.pakwheels.com/forums/members-member-rides/206788-my-blu-suzuki-swift-dlx-2012-a#post3677888