Hi .
It depends how many channels you want to use, and how much money!!
ACTion (see elswhere on mayhem) used to do a multichannel switcher unit that went on the rudder channel of your RC and could be used to switch on and off more than one item independently ( up to 8 items I think) only downside I could see was a slight delay on rudder response (should be no problem with most scale models), perhaps F L J can give an update on that.
Alternatively I have been using a fair copy of a circuit from
http://www.keirle8122.fsnet.co.uk/ modified to give me two isolated switch (relay ) outputs from a 'spare' channel on my 6channel RC
The components cost pence at Maplins, the most expensive item I bought being the servo plug and lead.
For the radar I have a home build circuit that starts the radar scanner when I 'start' the drive (propulsion) motor, and has a delay on switch off so the radar doesn't start stop when maneuvering/ going astern(backwards) but after not using engine for a while (adjustable up to about 1.5 min) the radar then stops
I know this is not how the purists would have it but I build my models for me not for others
Hope this is some help
Tom
ps I have no association with F L J :angel:
While I wrote this F L J posted