If I were planning to splice into an automotive wire under the hood of my car, obviously there are a lot of inductive and capacitive sources everywhere making a ton of noise, so I'm interested in a really good single conductor wire that is shielded so as not to introduce noise onto the lines I'm splicing into. Anyone know what it's called or have a link to an example? It's just a voltage tap so it doesn't need to carry power or have a thick gauge. Also, is there any way of dealing with the splice itself to shield it or is that pretty much impossible? Thanks.
Try searching for coax (coaxial) cable. Audio cables and RG (cable TV, etc.) cables are shielded.
If you're sensing a DC (or low frequency) signal you can use a parallel capacitor as a filter. And if the source happens to be low impedance that will help to reduce noise pickup too.
And for the splice itself, a metal box 
And NEVER connect the wire shield on both ends.
COAX I pretty much assumed. I was searching for it on google for a long time yesterday and finding all the wrong types… stuff for TVs, which I don’t want. I need some very thin gauge stuff that is outdoor rated, for autos.
As for grounding the sheild, you’re saying ground 1 end and not the other? Is that because of possible ground loops?
As for a metal box, basically you’re making a faraday cage around the junction? Grounded?
RG402 is good stuff.. and with suitable connectors ( SMA ) usable up to 18GHz.
Allan
It looks very robust and discrete enough in footprint to work well in my application. Thank you.
Gahhhrrrlic: As for grounding the sheild, you're saying ground 1 end and not the other? Is that because of possible ground loops?
Yes and yes. If you connect both ends you make an even better antenna.
omersiar: As for a metal box, basically you're making a faraday cage around the junction? Grounded?
Yes and yes again :)
I was a bit tongue in cheek about RG402. It's brilliant stuff for RF, but quite expensive, and won't take much DC current.
For signals - perfect. Probably well over the top.
Allan
I'll be using this cable to splice into various car sensors, which are practically zero current and usually 5V. They're all connected to the ECU anyway so they are very much signal regime so gauge is pretty much moot. I just don't want to break my car by adding noise.
USB cables are generally shielded. As are most microphone, audio, video, and cableTV coax cables. These are easy to find and cheap, unlike some of the exotic RF coax cables...
Good point. I recall noticing that when I chopped up a USB cable for a project many years ago. I suppose one would also need to consider the environment as well (vibration, heat, corrosion, etc) so even with so many options, the best choice wouldn't be so arbitrary. Do you have a recommendation that would be more economical than the previous recommendation (RG402) ? If I understand correctly, shielding is best for rejection of capacitive coupling but for EMI, twisted pair is best so a twisted pair inside a shield is the ultimate (CAT5?). Also multi-strand seems good for vibration in case 1 solid strand fatigues and breaks, although this isn't a catastrophic problem unless you're dealing with power and I'm not. Since I'm accessing many sensors in the car, the wires need to go to different areas so there's no chance I can exploit a multi-wire cable for multiple sensors... I'd need a dedicated line for each one, all routed back through the vehicle firewall.
Well I would consider a sensor wire breaking to also be catastrophic.
The cheapest wire on this page: http://www.aircraftspruce.com/catalog/elpages/edmotefzel.php has a single conductor and a shield. The Tefzel insulation is thin but very tough so it will fit in the same space as plain wire. It also takes high temperature, low pressure and anything in between.