1)obtain prior permission form the commissioner for using LED display screens on the sides of the car

2)for instance in Delhi In areas controlled by MCD outdoor advertisement devices visible to the public, are governed by provisions of section 142 to 146 of the Delhi Municipal Act. As per the provisions of the DMC Act, prior permission of the Commissioner is needed to display any advertisement to public view.

“Prohibition of erection, exhibition, fixation, retention or display of advertisement without written permission of the Commissioner — No person shall erect, exhibit, fix or retain or over any land, building, wall, hoarding, frame, post, kiosk/Bill Board (to be displayed on electric pole) or structure or upon or in any vehicle, any advertisement or display any advertisement to public view in any manner whatsoever in any place within the jurisdiction of the Corporation without the prior written permission of the Commissioner.

if it is commercial vehicle then you can do it after taking permission (paying municipal tax levied on advertisement) from the municipality. M V Act does not prohibits such ad but it should be advertised after taking proper permission.

There is no prohibition as per motor Vehicles Act, but such things will attract public distraction hence permission from office of Commissioner of police may be required so that you can safely drive your vehicle with the permitted advertisements in your car. You may clarify the details from the Commissioner's office about the size and other formalities in this regard.