the need for a relay is partially dependent on what type of switch you use and also the guage of the wire supplied in the harness kit. go with the chart for wire guage you need and check out the spec sheet for the accesory you are wiring up for the expected draw and then check that against the expected length of run (use a string to run out a fake expected wire run and then measure the string if you are unsure how long the run will be. add some to the length if anything so you don't under estimate the run length and get an excessive voltage drop in the end of your circuit). if the need comes up for the relay then the harness wire becomes the signal wire to turn on the relay and you run the requred wire guage from there forward to the accesory. you also need to run matching ground wire guage to a good ground andmake sure there are body, frame, engine ground cables in place. install all the relays in an easily accessed area but out of the way for road dirt and excessive heat. under the dash works well if you have the room because there is less chance things get wet. some guys will install the relays behind the glove box but that won't work if you plan on having a/c because that area will be spoken for. use double wall shrink tube on all the connections irregardless. get yourself a good pair of stripping/crimping pliers as well. I have a set of strippers and a seperate plier that crimps only. I like this because I don't like to have the crimper or stripper inside the handle area of the tool I like all the workings of the tool to be on the tip of the tool. sometimes that is important in a tight work area. there are some whizzy crimping tools out there with interchangeable tips etc but remember the basics, don't over strip, don't over crimp and try not to twist the wire after it is crimped because that causes strands to overlap each other inside the barrel of the connector and this can cause the wire strands to break. try not to use a crimper that only flattens the barrel of the connector because this usually allows the wire to pull out later. use double wall shrink tube over a bare terminal (I hardly ever use the connectors with the plastic covering simply because I always shrink tube anyway and also I like to see the quality of the crimp), tug on the connection after the criming but before the shrink tube. this will let you know if the crimp is holding.
you can also use a large junction terminal to bring big battery power to an area and junction from that to supply power to a relay block or an automatic reset circuit breaker block. you could use a maxi ATO style fuse at the battery end of that to protect that battery feed in case of a short in the circuit before the relays/breakers.
http://www.littelfuse.com/~/media/au...tion_chart.pdf
https://www.amazon.com/American-Auto.../dp/B005JUWZY0
I googled automotive relay block and this came up, looks interesting with an OEM looking box. it holds 12 relays which is overkill unless you have a whack of stuff to run.
https://www.online-led-store.com/switches-relays
or this from littelfuse
http://www.littelfuse.com/products/f...y-sockets.aspx
the whole littelfuse page
http://www.littelfuse.com/products/f...tribution.aspx
more stuff from amazon site
https://www.amazon.com/Pack-OLS-Rela...8&s=automotive